grub: directly load linked config file

While troubleshooting grub network boot issues,
I did some reading and found out our model of config
was technically wrong to use a menuentry to load config
in another menuentry which may or may not be loaded.

I mean it worked, but it is simpler to just say
"go source this content into our state".

Change-Id: I5e2ec2dc5110fa0a4f9e11478502a199354454f5

ironic/drivers/modules/initial_grub_cfg.template

@@ -1,7 +1,4 @@
-set default=initial
-set timeout=5
-set hidden_timeout_quiet=false
+echo Booting from $prefix
+
+source {{ tftp_root }}/$net_default_mac.conf
 
-menuentry "initial"  {
-configfile {{ tftp_root }}/$net_default_mac.conf
-}

ironic/tests/unit/common/test_pxe_utils.py

@@ -1248,7 +1248,7 @@ class TestPXEUtils(db_base.DbTestCase):
         def write_to_file(path, contents):
             self.assertIn('/grub/grub.cfg', path)
             self.assertIn(
-                'configfile /tftpboot/$net_default_mac.conf',
+                'source /tftpboot/$net_default_mac.conf',
                 contents
             )
 

releasenotes/notes/grub-default-template-change-a7bacdef4543a9ae.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixes the default grub initial configuration to be simpler and directly
+    load the generated configuration file. The template also includes output
+    which also help operators understand the context as to where the node is
+    booting from, should issues be encountered.