Signal a build failure for kernel_tests

Always write exit code to /proc/exitcode so that the builder can
fail a build if any of the tests fail by reading this return value
which will be the return from the build script.

BUG=23016816
TEST=Ran locally and check output of "echo $?" is not 0 when a test fails

Change-Id: I5f38c7c844a17773a9459746c3b8704b9b4bd7fd