The zipl bootloaders are s390x specific, so only build them if the
`HOST_ARCH' is set to s390x.
While at it, rename `INC_FILES` to `EMBEDDED_BOOTLOADERS`. Also
introduce `EXTERNAL_BOOTLOADERS` variable and use it in the `install`
Makefile target.
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>