s390/boot: Use strspcy() instead of strcpy()

Convert all strcpy() usages to strscpy(). strcpy() is deprecated since
it performs no bounds checking on the destination buffer.

Reviewed-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>