AVR: Back out experimental MEMX build changes.  Although they do work, the cause other compilictions.

mkdeps.sh and mkdeps.bat have been deleted.  All configurations must now use the compiler mkeps prograqm

All AVR toolchain configuration names must end with _TOOLCHAIN to work properly with nuttx/tools/testbuilds.sh

ATMEGA2560: Introduce ostest variant for Arduino Mega2560

sim/nsh: hello task sometimes needs a larger stack size or things get clobbered when calling into the system write() interface