initial attempt at documenting how to port uClibc

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+Some notes to help future porters.  Replace 'ARCH' with whatever arch 
+you are hacking on.
+
+====================
+=== Config Files ===
+====================
+- create extra/Configs/Config.ARCH
+   See the other arch files for some good examples.  powerpc/sparc/alpha 
+   should be pretty simple templates.
+- add ARCH to the 'Target Architecture' list in extra/Configs/Config.in
+- for now, disable shared libraries
+   add HAVE_NO_SHARED and ARCH_HAS_NO_LDSO to Config.ARCH's HAVE_ELF
+
+====================
+=== libc sysdeps ===
+====================
+(note: if glibc has already been ported to your arch, you can usually just 
+       copy a lot of files from them rather than coding from scratch)
+- create libc/sysdeps/linux/ARCH
+- copy Makefile and Makefile.arch from libc/sysdeps/linux/i386/
+- set CSRC and SSRC to nothing in Makefile.arch for now
+
+- create crt1.S which defines the _start function ... you will probably want
+  to clear the frame pointer to make gdb happy, and then you will want to call
+  the funcion __uClibc_main() which takes these parameters:
+   __uClibc_main(main(), argc, argv, _init(), _fini())
+  for now, you can disable the UCLIBC_CTOR_DTOR option and just set those two
+  arguments to NULL
+  glibc stores this function in libc/sysdeps/ARCH/elf/start.S
+
+- create these additional files in ARCH/bits/
+
+  (template versions can be found in common/bits/ for you to tweak)
+  endian.h  fcntl.h  setjmp.h  stackinfo.h  uClibc_arch_features.h  wordsize.h
+
+  kernel_types.h should be created based upon linux asm-ARCH/posix_types.h
+
+  copy linux asm-ARCH/stat.h to bits/kernel_stat.h
+
+  create syscalls.h based upon linux's unistd.h / glibc's sysdeps.h ... really
+  you just want to define the _syscall[0-6] macros
+
+- at this point, you should have enough to generate a working HELLO WORLD
+  static binary (see test/silly/*.c files)
+
+- if you want UCLIBC_CTOR_DTOR support, you will need to create crti.S and
+  crtn.S files which define function prologues/epilogues
+
+- for a more stable static port, you will need to create these files (and
+  update the Makefile.arch values accordingly)
+    __longjmp  bsd-_setjmp  bsd-setjmp  brk  clone  setjmp  syscall  vfork
+  usually these are written in assembler, but you may be able to cheat and
+  write them in C ... see other ports for more information
+
+====================
+=== pthread deps ===
+====================
+
+TODO: nptl / linuxthreads / linuxthreads.old
+
+====================
+=== ldso sysdeps ===
+====================
+- enable ldso/shared options in your extra/Configs/Config.ARCH file
+- you'll need to create these files in ldso/ldso/ARCH/
+  dl-debug.h  dl-startup.h  dl-syscalls.h  dl-sysdep.h  elfinterp.c  resolve.S
+
+- dl-debug.h: define string versions of all the relocations of your arch in the
+  _dl_reltypes_tab array ... the index should match the actual reloc type, so
+  if the value of say R_X86_64_PC16 is 13, then "R_X86_64_PC16" better be at
+  index 13 of the array
+
+- dl-startup.h:
+  - define the _start function which should call _dl_start which takes just one
+    parameter ... a pointer to argc (usually on the stack)
+  - define the GET_ARGV() macro which calculates the value of argv based upon
+    the parameter passed to _dl_start (usually it's simply just ARGS+1)
+  - define PERFORM_BOOTSTRAP_RELOC() macro which will handle just the relocs
+    that the ldso itself will generate
+
+- dl-syscalls.h:
+  if you wrote your bits/syscalls.h file correctly in the libc step above, you
+  can simply copy this file from another arch and be done ... otherwise you
+  will have to define the syscall[0-6] macros again, but this time setting
+  _dl_errno instead of just errno
+
+- dl-sysdep.h:
+  misc cruft goes in here ... you want to:
+  - either define or undefine ELF_USES_RELOCA
+  - define the INIT_GOT macro
+  - define MAGIC1 to the EM_### value your ELF arch uses
+  - define ELF_TARGET to a string name for your arch
+  - define the do_rem() macro
+  - define misc ALIGN macro's
+  - define elf_machine_type_class() macro
+  - define the inline functions elf_machine_dynamic, elf_machine_load_address,
+    and elf_machine_relative
+  glibc stores a bunch of these values in libc/sysdeps/ARCH/dl-machine.h
+
+- elfinterp.c:
+  define all the relocation functions ... it's best if you just copy from
+  another arch which uses the same type of relocations (REL or RELA) and
+  start from there.
+
+- resolve.S:
+  front end of lazy relocation ... define the _dl_linux_resolve symbol which
+  is called by a PLT entry which has yet to be setup ... you will want to:
+  - set up arguments for _dl_linux_resolver()
+  - call _dl_linux_resolver()
+  - clean up after call
+  - jump to function address now stored in PLT