pc-bios/s390-ccw: fix loadparm initialization and int conversion

pc-bios/s390-ccw: fix loadparm initialization and int conversion

Rename the loadparm char array in main.c to loadparm_str and
increased the size by one byte to account for a null termination
when converting the loadparm string to an int  via atoui. We
also allow the boot menu to be enabled when loadparm is set to
an empty string or a series of spaces.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reported-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>