AndroidKernel: Look for defconfig under vendor/

The target configuration searches for defconfig under
configs/vendor/ by default. Hence, change the search
path for the same for kernel configuration.

Change-Id: Ica83363fc6783eb1ce2d46acfd755a1915e1d113
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>