Add `TARGET_ARCH_<ARCH>` for each architecture (e.g. `TARGET_ARCH_CORTEX` for
Cortex M) to allow users to overwrite the target triple for a specific arch
from ~/.profile or ~/.bashrc (or the like) without overwriting it for all others
as well.
- Moved compiler & linker flags from boards/common/msba2 to cpu/arm7_common
- Moved dependency to newlib nano to cpu/arm7_common
- Moved config to link in cpu/startup.o to cpu/arm7_common