scripts/prune-kernel: Use kernel-install if available

If the new-kernel-pkg utility isn't present, try using kernel-install.
This is what the %preun scriptlet in scripts/package/mkspec does too.

Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>