s390/ipl: use kstrtobool() instead of strtobool()

strtobool() is the same as kstrtobool().
However, the latter is more used within the kernel.

In order to remove strtobool() and slightly simplify kstrtox.h, switch to
the other function name.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/58a3ed2e21903a93dfd742943b1e6936863ca037.1673708887.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>