simulator: fix serial_write

printf("%s") will print a NULL terminated string, while rt_serial_write
should print buffer for size length. This also fix segfault when the
string is not null terminated but specified by size.
Merge pull request #87 from grissiom/static-usbd

Static usbd
Merge branch 'master' of https://github.com/RT-Thread/rt-thread into ForPullRequest
add missing dfs_mkfs prototype in dfs_fs.h
add usb state
format code by Astyle.
move initial alarm event & mutex in system init.
fix spelling error.
add library file for Keil MDK project generation
Merge pull request #91 from aozima/pulls

Pulls
Merge pull request #89 from grissiom/dfs-misc-fix

add missing dfs_mkfs prototype in dfs_fs.h
Merge pull request #90 from heyuanjie87/ForPullRequest

For pull request
Merge pull request #88 from grissiom/fix-sim-serial

simulator: fix serial_write