AndroidKernel: Explicitly mark certain targets as phony

kerneltags and kernelconfig are phony targets, so mark them
with .PHONY.

Change-Id: If2b6d9c20f8954f217c68dd8dec2360592f74492
Signed-off-by: Divya Sharma <divyash@codeaurora.org>
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>