atexit() functions now called when task killed by task delete; For MCUs with...

atexit() functions now called when task killed by task delete; For MCUs with <= 64Kb of SRAM, CONFIG_MM_SMALL can be defined to reduce the memory allocation overhead

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3648 42af7a65-404d-4744-a932-0658087f49c3