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 <[e-mail]>
Signed-off-by: Marc Hartmayer <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/boot/install.sh

···
 
 echo "Warning: '${INSTALLKERNEL}' command not available - additional " \
      "bootloader config required" >&2
-if [ -f $4/vmlinuz-$1 ]; then mv $4/vmlinuz-$1 $4/vmlinuz-$1.old; fi
-if [ -f $4/System.map-$1 ]; then mv $4/System.map-$1 $4/System.map-$1.old; fi
+if [ -f "$4/vmlinuz-$1" ]; then mv -- "$4/vmlinuz-$1" "$4/vmlinuz-$1.old"; fi
+if [ -f "$4/System.map-$1" ]; then mv -- "$4/System.map-$1" "$4/System.map-$1.old"; fi
 
-cat $2 > $4/vmlinuz-$1
-cp $3 $4/System.map-$1
+cat -- "$2" > "$4/vmlinuz-$1"
+cp -- "$3" "$4/System.map-$1"