Signed-off-by: NMurphyZhao <d2014zjt@163.com>

Update license for RT-Thread kernel.

rename _errno to __rt_errno for conflict in WIN32.

1. Add signal implementation;
2. Change the 'rt_uint8_t' of cmd to 'int';

Update year info from 2016 to 2017

chang /*@{*/ to /**@{*/ and /*@}*/ to /**@}*/

in rt_console_set_device, open new device first and get the new device to _console_device.
我在编写程序时，在device驱动中打印log，但是使用rt_kprintf时，如果_console_device存在非空，那就执行驱动的write函数，但是这是device还没有初始化成功，但是已经提前给_console_device赋值，导致了程序无故fault，以上述修改可避开这样的矛盾。

Commit a6251a43 changed the serial device to check open_flag
instead of flag. So we should change the rt_kprintf counterpart as well.

vsnprintf is a common string function that could be used in many places.
Using both vsnprintf in libc and vsnprintf in the RTT could make a
bigger image. Moreover, if newlib is not enabled when compiling with
GCC, referencing vsnprintf will lead to link error:

    .../arm-none-eabi/lib/armv7-ar/thumb/softfp/libc.a(lib_a-sbrkr.o):
    In function `_sbrk_r':
    sbrkr.c:(.text._sbrk_r+0xc): undefined reference to `_sbrk'
    collect2: error: ld returned 1 exit status

Using rt_vsnprintf could avoid such problem.

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