SBP now using libswiftnav SBP implementation.

 - All callbacks updated to follow new function prototype
 - screaming_death updated to use libswiftnav SBP functions too
 - NAP hash checked from init() after sbp_setup so screaming_death
   works if hash check fails
 - Flash callbacks updated to return an error code in the
   MSG_FLASH_DONE message instead of calling screaming_death.
 - flash.py checks MSG_FLASH_DONE return code but doesn't attempt
   to handle errors properly
Merge branch 'master' of github.com:swift-nav/piksi_firmware into new_sbp
Updated all tests for new SBP code. Everything building.
Update Makefile so libopencm3 is only built for F4.