build: core: Parameterize recovery with a variant

Include extra vendor config early.  If RECOVERY_VARIANT is set, use it
to look for the recovery in an alternate path.

Change-Id: Iba54f0746d728c0dfeeb67b4d3166d5e25b1b5d7

Conflicts:
	core/pathmap.mk

	modified:   core/pathmap.mk
Re-evaluate the pathmap if there's a recovery variant in play

Chances are the recovery location has changed, so rebuild the pathmap
to compensate. This lets the variable be defined in BoardConfig files
instead of the global environment

Change-Id: I87c23a452656ffa7817711796c17f79e30465756

Conflicts:
	core/config.mk

	modified:   core/config.mk