Build system_dlkm as ext4 the same way android system does.
At the same time modify the build to mirror how vendor_dlkm
is built.

Test: build with BUILD_KERNEL=1, flashed and booted.

Signed-off-by: Andrew Chant <achant@google.com>
Bug: 236753150
Change-Id: If92c8f8426fd16636113bbdc09fd55001aeec6f0