mxgrub: Add --set-default to switch default kernel

Add new command `mxgrub --set-default {label}` which sets the default
kernel. It is not automatically selected for the next boot.

Note, that the designation of the default kernel is via the symlink
/boot/bzImage.x86_64, which is distributed by the distmaster.

Resolves #74

mxgrub/mxgrub

@@ -12,6 +12,7 @@ sub USAGE { return <<"EOF" }
     $0 --initramfs : update /boot/grub/initramfs.igz from /project/admin/initramfs
     $0 --test      : perform miscellaneous tests (VX50 board)
     $0 --reboot    : attempt kexec reboot of selected kernel
+    $0 --set-default {label} : select this kernel as the default kernel
 EOF
 
 sub sys {
@@ -363,17 +364,27 @@ sub cmd_reboot {
 	sys('kexec',"/boot/$image",'--initrd=/boot/grub/initramfs.igz',"--command-line=root=LABEL=root $KERNEL_PARAMETER");
 }
 
+sub cmd_set_default {
+	my ($label)=@_;
+	my $image=label_to_image($label);
+	-e "/boot/$image" or die "/boot/$image: no such file\n";
+	sys 'ln','-sf',$image,'/boot/bzImage.x86_64';
+	scan_mariux(0);
+	update_grub_cfg();
+}
+
 umask 022;
 check_grub_installation();
 
-our ($opt_list,$opt_update,$opt_initramfs,$opt_test,$opt_reboot);
+our ($opt_list,$opt_update,$opt_initramfs,$opt_test,$opt_reboot,$opt_set_default);
 
 GetOptions(
 		'list'      => \$opt_list,
 		'update'    => \$opt_update,
 		'initramfs' => \$opt_initramfs,
 		'test'      => \$opt_test,
 		'reboot'    => \$opt_reboot,
+		'set-default' => \$opt_set_default,
 
 ) or die USAGE;
 
@@ -407,6 +418,9 @@ if ($opt_list) {
 } elsif ($opt_reboot) {
 	@ARGV==0 or die USAGE;
 	cmd_reboot();
+} elsif ($opt_set_default) {
+	@ARGV==1 or die USAGE;
+	cmd_set_default($ARGV[0]);
 } else {
 	@ARGV==1 or die USAGE;
 	cmd_install($ARGV[0]);