mkvendor: fix echoed build commands

cm.mk.template uses PRODUCT_NAME := cm___DEVICE__

Change-Id: I063fc4b93d90ebb034637bf3e92d83c77aa439ca
build: Add support for extra kernel build variables

 * This allows specifying a variant defconfig, and an selinux defconfig
   which are simply configuration fragments.

Change-Id: I97882ae3b8c2e16ff6a7dce8dd3a70d70f8aa866
Merge remote-tracking branch 'upstream/cm-10.1' into cm-10.1