* [rtdef] rename RT_WEAK attribute as rt_weak

* [rtdef] rename RT_USED attribute as rt_used

* [rtdef] rename RT_SECTION attribute as rt_section

* [rtdef] rename ALIGN attribute as rt_align

* [legacy] add RT_USED ALIGN RT_SECTION RT_WEAK as legacy support

* Synchronize the code of the rt mart branch to the master branch.
  * TTY device
  * Add lwP code from rt-smart
  * Add vnode in DFS, but DFS will be re-write for rt-smart
  * There are three libcpu for rt-smart:
    * arm/cortex-a, arm/aarch64
    * riscv64
Co-authored-by: NRbb666 <zhangbingru@rt-thread.com>
Co-authored-by: Nzhkag <zhkag@foxmail.com>

* Fix some compilation warning

* 补充修正一些数据类型的使用错误
Co-authored-by: Meco Man <920369182@qq.com>

* correct descriptions

correct descriptions in ipc.c and timer.c
Co-authored-by: NStanley <stanleylwinn@gmail.com>

* [add] the function realization of signal for posix.
* [update] the posix support for armclang.
* [add] the new macro "RT_USING_POSIX_TIMER".
* [modify] select "RT_USING_SOFT_TIMER" when use posix'timer.
* [bug] optimize the logic for the "time_xxx" functions.
* [modify] use "RT_USING_POSIX_TIMER" to protect the macro definition.
* [modify] error code when except happened.
* [delete] the "environ" is useless at this time.

Signed-off-by: NWillianChan <chentingwei@rt-thread.com>

[kernel]Change 'rt_timer_timeout_hook' function to 'rt_timer_enter_hook' and add 'rt_timer_exit_hook' hook function.

Update license for RT-Thread kernel.