mxgrub: Do not use old command line on reboot

When we use `mxgrub --reboot` we don't want to reuse the previous kernel
command line.

Generate a new command line for kexec.

This way, changes we make to $KERNEL_PARAMETER in this script can be activated
by `mxgrub --reboot` as well.

mxgrub/mxgrub

@@ -360,7 +360,7 @@ sub cmd_reboot {
 	my $image=label_to_image($chosen);
 	-e "/boot/$image" or die "/boot/$image: no such file\n";
 	sys('sync');
-	sys('kexec',"/boot/$image",'--reuse-cmdline','--initrd=/boot/grub/initramfs.igz');
+	sys('kexec',"/boot/$image",'--initrd=/boot/grub/initramfs.igz',"--command-line=root=LABEL=root $KERNEL_PARAMETER");
 }
 
 umask 022;