add libcpu/sim/posix/cpu_port.c
add bsp: simluator for linux
add some comments and clean code in cpu_port.c
re-write the rt_hw_interrupt_enable/disable
add finsh, but still cannot work, only can be built with gcc
surport finsh
add gcc.ld
rewrite cpu_port.c (but still has bug)
fix bug in cpu_port.c(can work very well)
clean code in cpu_port.c, add some comments
recover the terminal's behaviour when runing exit in finsh
fixed the string descriptor send to host more than actual
more deterministic on timer

If two timer will timeout at the same tick, the one started later will
be called later. I've tested the patch on simulator and it _seems_ OK.

Reported-by: xdzy on the forum and delin17 <delin17@qq.com>
Merge pull request #16 from grissiom/fix-timer

more deterministic on timer
Merge pull request #14 from heyuanjie87/ForPullRequest

fixed the string descriptor send to host more than actual
Use customized command string in scons
use sd as '/' mount ponit
clean code in sd_sim.c to make sure it can be built with gcc in linux
enable FAT and uffs in simlinux/rtconfig.h
fix bug in finsh when built with 64bit-gcc