Prevent include of execinfo header

This header does not exists for cross compiliations (at least
using the arm-none-eabi from launchpad)
Add simple to_string for RIOT cross compiling
Removed redundant no compiler check flag
Add additional RIOT include path
Add board specific include path
Exclude gcc specific hacks

These functions are implemented in the RIOT specific part
of the thread header.
Add quick hack

Needs better handling later on.
Add fix for mac_address on stm32f4discovery

Fix for our current testing board only. RIOT will offer an implementation
in the future.