mpc: conditional compilation for comm interfaces

Conditional compilation for the phasor_comm USART and MPC TWI
interfaces. In theory it compiles one or both based on MPC_TWI and
PHASOR_COMM_USART... It's fucking ugly.
mpc: include serialcomm.h with phasor_comm.h
mpc: fix a memleak if CRC fails
mpc: fix a logic error that prevented pkts from sending

Fixes an issue that begin with 4cf1605299c06df111acf15cf2e94eb2d52cc562,
where the mpc master would not transmit any messages.
Merge branch 'master' into comm-rewrite

Conflicts:
	include/mpc.c
mpc: change twi address mask

This puts the phasor in the address mask used by the chest, shoulder,
back. Even though the phasor is not on the TWI bus, this allows those
boards to receive TWi transactions with the phasor address bit set.
mpc: fix typo

Typo in MPC_TWI_SLAVE_ISR caused the ISR not to run