s390/boot: Install bzImage as default kernel image

The compressed kernel image is built since commit "[S390] add support for
compressed kernels" (1844c9bc).
Now install the compressed kernel image (bzImage) as default.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>