build: core: Parameterize recovery with a variant
WE ADDED: https://github.com/CyanogenMod/android_build/commit/6b21727ef8997762d83691cb453aa950aa8d574b
...but left out this commit

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

Change-Id: Iba54f0746d728c0dfeeb67b4d3166d5e25b1b5d7