gpt: new gptprio.next command for selecting priority based partitions

Basic usage would look something like this:

    gptprio.next -d usr_dev -u usr_uuid
    linuxefi ($usr_dev)/boot/vmlinuz mount.usr=PARTUUID=$usr_uuid

After booting the system should set the 'successful' bit on the
partition that was used.

grub-core/Makefile.core.def

@@ -831,6 +831,11 @@ module = {
   common = commands/gptrepair.c;
 };
 
+module = {
+  name = gptprio;
+  common = commands/gptprio.c;
+};
+
 module = {
   name = gpt;
   common = lib/gpt.c;