* [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.
* [kernel][mem] Multiple instances of small memory allocation algorithm
* [kernel][mem] Change small memory management algorithm memory header flag
* [kernel][mem] Fix assertion problem
* [kernel][slab] Multiple instances of slab memory management algorithm
* [kernel][memheap] Remove rt_malloc/rt_free/rt_realloc and other related memory interfaces
* [kernel][mem] Clean up memory space of small memory management objects
* [kernel][kservice] Add memory application interface and thread protection interface
* [kernel][kservice] Fix function return value problem
* [kernel][memheap] Optimize memheaptrace print
* [kernel][memheap] Support best mode
* [kernel][memory] Remove semaphore lock
* [kernel][memheap] Add locked flag
* [kernel][memory] Support malloc memory in interrupt
* [kernel][memheap] Add 'memheapcheck' cmd
* [kernel][mem] Fix failure to request full memory
* [kernel][memheap] Fix compilation warning
* [kernel][mem] Fix mem realloc ASSERT
* [examples][testcases] Add small mem testcase
* [examples][mem_tc] Modify test memory size
* [examples][testcases] Add slab memory management algorithm test case
* [examples][testcases] fix small memory management algorithm test case
* [kernel][memory] Adjusting memory allocation algorithm object definition and interface
* [kernel][memory] Fix compilation warning
* [examples][utest] Fix mem test case
* [examples][utest] fix slab test case
* [utest][testcases] Shorten test time
* [kernel][memory] Formatting code
* [examples][utest] Adjust test run time
* [examples][utest] Formatting code
* [bsp] update all rtconfig.h