Move sbp_utils to libswiftnav

[ljbade]

Came up today when adding a new function to handle lock time in the new SBP messages that the sbp_utils.c should be in libswiftnav so it can get some decent unit tests.

/cc @benjamin0 @ejconlon @denniszollo

[jacobmcnamee]

That would require pulling libsbp into libswiftnav, no?

[ljbade]

@jacobmcnamee Oh, hmm. Maybe libsbp might be a better location for the utility functions then