Add an execution check for BOARD_KERNEL_VERSION

Make sure the specified BOARD_KERNEL_VERSION matches what we extract
from the kernel binary. As other parts of the build system might rely on
the kernel version.

Test: set BOARD_KERNERL_VERSION to foo, make sure build fails
Bug: 293313353
Change-Id: I9005d338fcc3dbb692a6ea7a518a5e63219b2c99