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
Merge branch 'kk' of https://github.com/CarbonDev/android_build into kk