new file:   apps/echo.c
new file:   apps/makefile
new file:   apps/start.S
    application directory.
print echo
add .code32
link start.o first; add echo.bin rule
add exec(); test exec()
add k_assert.h
new file:   include/mm.h
new file:   mm/makefile
new file:   mm/mm.c
    a very simple memory manage.
modified:   start.c
    add mm_init.
    remove memtest to mm.c
    test alloc_mem()
add mm rule
modified:   fs/romfs.c
    romfs_namei():
      use alloc_mem() instead of buf[512]
      read all romfs ramdisk by sb->device->storage_size.
      use alloc_mem() to alloc memory for inode.
modified:   include/process.h
modified:   process.c
    change exec() prototype.
    remove test romfs code to start.c .
ramdisk_dout():
  return read size.

fill ramdisk_sd.storage_size by real romfs size.
add test romfs/exec() code
add elf support
add color
change next_offset type from u8 to u32
new file:   q.sh
    qemu gdb remote debug command.
add KERNEL_INFO macro, get kernel address that is loaded
  by kernel loader.
add struct Elf32_Shdr and needed macro
add get_kernel_info()