distro/rhel90: use unified grub config

Feodra 34 and thus RHEL 9 switched to a unified grub configuration,
which means that the main grub config is always located in the same
location, /boot/grub2/grub.cfg.[1] osbuild has used this scheme for
hybrid boot on x64 but not on pure efi systems like aarch64. The
new osbuild option `uefi.unified` was introduced to select that new
unified grug cfg scheme also for those, pure efi, systems. Use that.

[1] https://fedoraproject.org/wiki/Changes/UnifyGrubConfig

internal/distro/rhel90/stage_options.go

@@ -332,7 +332,8 @@ func grub2StageOptions(rootPartition *disk.Partition, bootPartition *disk.Partit
 
 	if uefi {
 		stageOptions.UEFI = &osbuild.GRUB2UEFI{
-			Vendor: "redhat",
+			Vendor:  "redhat",
+			Unified: legacy == "", // force unified grub scheme for pure efi systems
 		}
 	}
 

test/data/manifests/rhel_90-aarch64-ami-boot.json

@@ -1018,6 +1018,7 @@
               "boot_fs_uuid": "0194fdc2-fa2f-4cc0-81d3-ff12045b73c8",
               "kernel_opts": "console=ttyS0,115200n8 console=tty0 net.ifnames=0 rd.blacklist=nouveau nvme_core.io_timeout=4294967295 iommu.strict=0 crashkernel=auto",
               "uefi": {
+                "unified": true,
                 "vendor": "redhat"
               },
               "saved_entry": "ffffffffffffffffffffffffffffffff-5.14.0-0.rc3.29.el9.aarch64"
@@ -9920,4 +9921,4 @@
     },
     "timezone": "UTC"
   }
-}
+}

test/data/manifests/rhel_90-aarch64-qcow2-boot.json

@@ -920,6 +920,7 @@
               "root_fs_uuid": "0194fdc2-fa2f-4cc0-81d3-ff12045b73c8",
               "kernel_opts": "console=tty0 console=ttyS0,115200n8 no_timer_check net.ifnames=0 crashkernel=auto",
               "uefi": {
+                "unified": true,
                 "vendor": "redhat"
               },
               "saved_entry": "ffffffffffffffffffffffffffffffff-5.14.0-0.rc3.29.el9.aarch64"
@@ -9866,4 +9867,4 @@
     },
     "timezone": "New_York"
   }
-}
+}