cortexm-common: minimal configuration grew by 200 bytes

[miri64]

Checking our minimal configuration I noticed that it grew by 200 bytes (at least on IoT-LAB-M3, for Cortex-M0 boards even more IIRC) since we last checked and documented it (in this case in some paper draft ;-)). After some bisecting I found 4bfce89 to be the commit that significantly made it bigger (though there are others that always made it bigger some bytes). This particular commit unified the periph initialization. Due to that SPI is included, though it isn't really needed. Just another example of our messed-up build system maintenance ;-).

(This is just a note for @haukepetersen, we discussed this already offline)

[roberthartung]

True.. I also came upon the periph_init() call in cpu_init and thought if there was a better solution to this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.