g2: Cleanup BoardConfigCommon.mk

Who the hell thought some of these flags were a good idea? There's
stuff in here from ARMv5 (I mean, really???). Trash all the
useless flags and configs and use something that vaguely resembles
the makefile for a device from this decade...

Change-Id: I5faa32cd2ddfcfadb45253946923e1667e449cc2