m68k/kernel: replace strncpy() with strscpy()

Swapped out strncpy() for strscpy() in parse_uboot_commandline() while
copying to commandp. strscpy() makes sure the string is properly null-
terminated and gives a more useful return value so it's just a safer
choice overall.

Link: https://github.com/KSPP/linux/issues/90


Signed-off-by: Mohammad Mahdi Anbaraki <m.mahdianbaraki@gmail.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>