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.

Makefile.util.def

@@ -1156,6 +1156,12 @@ script = {
   common = tests/gptrepair_test.in;
 };
 
+script = {
+  testcase;
+  name = gptprio_test;
+  common = tests/gptprio_test.in;
+};
+
 program = {
   testcase;
   name = example_unit_test;