s390/boot: improve install.sh script

Use proper quoting for the variables and explicitly distinguish between
command options and positional arguments.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>