mxgrub: Always install GRUB in MBR

Contradicting the documentation, that `i386-pc` is the default target,
on UEFI systems it is not.

    $ sudo grub-install --boot-directory=/boot /dev/nvme0n1
    Installing for x86_64-efi platform.
    grub-install: error: cannot find EFI directory.

So, pass `--target=i386-pc` to install GRUB in the MBR.

    $ sudo grub-install --target=i386-pc --boot-directory=/boot /dev/nvme0n1
    Installing for i386-pc platform.
    Installation finished. No error reported.

mxgrub/mxgrub

@@ -313,18 +313,18 @@ sub cmd_install {
 	if (is_efi) {
 		sys 'mount','-L','ESP','/boot/efi' and exit 1;
 		if (-d '/boot/efi/EFI/grub/grubx64.efi') {
-			print "GRUB is already installed.\n";
+			print "GRUB for UEFI is already installed.\n";
 		}
 		else {
 			sys 'grub-install','--target=x86_64-efi',$root_disk and exit 1;
 		}
 		sys 'umount','/boot/efi' and exit 1;
+	}
+
+	if ($mbr_type ne 'GRUB') {
+		sys 'grub-install','--target=i386-pc',$root_disk and exit 1;
 	} else {
-		if ($mbr_type ne 'GRUB') {
-			sys 'grub-install',$root_disk and exit 1;
-		} else {
-			print "GRUB is already installed.\n";
-		}
+		print "GRUB is already installed in MBR.\n";
 	}
 
 	if ($label eq 'default') {