cortex m3: add cortex_m3 reset_config cmd

cortex m3: add cortex_m3 reset_config cmd

This new cmd adds the ability to choose the Cortex-M3
reset method used.
It defaults to using SRST for reset if available otherwise
it falls back to using NVIC VECTRESET. This is known to work
on all cores.

Move any luminary specific reset handling to the stellaris cfg file.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>