[pthreads] Fix pthread_cond_init issue. #261
[libc] fix compiling warning
[external] PNG_NO_WRITE_SUPPORTED issue

Move PNG_NO_WRITE_SUPPORTED outside of RT_USING_NEWLIB. #197
Fix some spell error;
add more SPI device driver.
Merge pull request #308 from aozima/pulls

[DeviceDrivers] Add more SPI drivers.
[DFS] make comments cleanup.
Merge branch 'master' of https://github.com/RT-Thread/rt-thread
Merge branch 'master' of https://github.com/RT-Thread/rt-thread