Revert "Allow disabling DroidDoc build."

This reverts commit 875e83165081425e74ce7c999b51c01cac0c876c

Change-Id: I670a1bea555cc7ac8346fba347b9cb40092c3da1
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