EGPRS Type 1, 2, and 3 headers are used by OsmoPCU and OsmoBTS.
Move the header definitions to libosmocore to be shared by both
packages.
Modify the struct variable naming to use *_hi/*_lo instead of
*_a/*_b in order to be consistent with existing naming used in
libosmocore.
Change-Id: I98687ad981d27502aec42729611937ba1caf207c