mxgrub: Relax VX50 check

We sucessfully switched six VX50 systems to GRUB. All of these have a
BIOS 2.* version. Only VX50 systems with a BIOS 3.* version were unable
to boot with GRUB.

Once a system has been switched to GRUB, there is no known risk
with this tool to reinstall GRUB or select another kernel.

Do not warn or bail out on VX50 systems, which were already switched to
GRUB.

mxgrub/mxgrub

@@ -283,13 +283,15 @@ sub cmd_list {
 
 	$mbr_type eq 'GRUB' or warn "WARNING: $root_disk MBR type is $mbr_type\n";
 
-	is_VX50() and warn "WARNING: This is a VX50, I refuse to install grub here\n";
+	is_VX50() && $mbr_type ne 'GRUB' and warn "WARNING: This is a VX50, I refuse to install grub here\n";
 }
 
 sub cmd_install {
 	my ($label)=@_;		#
+	my ($root_device,$root_disk)=root_dev();
+	my $mbr_type=check_mbr($root_disk);
 
-	is_VX50() and die "This is a VX50, I refuse to install grub here\n";
+	is_VX50() && $mbr_type ne 'GRUB' and die "This is a VX50, I refuse to install grub here\n";
 
 	if ($label eq 'default') {
 		scan_mariux(0);
@@ -301,8 +303,6 @@ sub cmd_install {
 
 	update_grub_cfg();
 
-	my ($root_device,$root_disk)=root_dev();
-	my $mbr_type=check_mbr($root_disk);
 	if ($mbr_type ne 'GRUB') {
 		sys 'grub-install',$root_disk and exit 1;
 	}