pc-bios/s390-ccw: rename MAX_TABLE_ENTRIES to MAX_BOOT_ENTRIES

pc-bios/s390-ccw: rename MAX_TABLE_ENTRIES to MAX_BOOT_ENTRIES

The MAX_TABLE_ENTRIES constant has a name that is too generic. As we
want to declare a limit for boot menu entries, let's rename it to a more
fitting MAX_BOOT_ENTRIES and set its value to 31 (30 boot entries and
1 default entry). Also we move it from bootmap.h to s390-ccw.h to make
it available for menu.c in a later patch.

Signed-off-by: Collin Walling <walling@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>