[shell] Fix compiling issue when DFS is not enable
[Kernel] comments clean up.
[libcpu] Add builtin ffs implementation for Cortex-M4.
[Kernel] comments clean up.
[LogTrace] Use OFLAG_RDWR to replace FLAG_RDWR when opening a device.
[Kernel] Use stream mode to open console device.
Merge branch 'master' of https://github.com/RT-Thread/rt-thread