Makefile (APPS): we don't need shell-ping and shell-time here
platform/stm32-e407/shell-wpan.h: workaround for build dependency

Fix this properly later.
platform/{freertos,stm32-e407}/: -Wno-strict-aliasing -fno-strict-aliasing

Contiki plays lots of aliasing-through-union tricks, supposedly to
obtain tighter code. GCC complains bitterly about these if -Wall is
set, even though they may be safe.

Rather than trying to sanitize the code in question, we just disable
the offending warning along with the underlying optimization.