kbuild/mkspec: support 'update-bootloader'-based systems

When uninstalling kernel RPM, we're unconditionally calling
"new-kernel-pkg --remove". This is useless on systems which are based on
'update-bootloader' script instead.

Support update-bootloader removal method as well in case the script is
present; contrary to new-kernel-pkg, this needs to be done in %postun,
otherwise update-bootloader will refuse to remove entry for kernel for
which the binary still exists.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Michal Marek <mmarek@suse.com>