Use architecture-specific grub2 RPMs on RHEL8

RHEL8 ships a bunch of grub2-efi-X-modules in its main
repository, each of which provides grub2-efi-modules,
potentially causing nondeterminism when building images.

This changes the DIB elements to always use architecture-
specific RPMs when RHEL8 is selected.

Change-Id: If94f3721195d5ecd80036e4234a3ca223a19c349
Related: https://bugzilla.redhat.com/show_bug.cgi?id=1716672

diskimage_builder/elements/bootloader/pkg-map

@@ -1,4 +1,17 @@
 {
+  "release": {
+    "rhel": {
+      "8": {
+        "extlinux": "syslinux-extlinux",
+        "grub-pc": "grub2-tools grub2",
+        "grub-efi-amd64": "grub2-tools grub2-efi-x64 grub2-efi-x64-modules",
+        "grub-efi-arm64": "grub2-tools grub2-efi-aa64 grub2-efi-aa64-modules",
+        "grub-efi-aarch64": "grub2-tools grub2-efi-aa64 grub2-efi-aa64-modules",
+        "grub-efi": "grub2-tools grub2-efi",
+        "grub-ppc64": "grub2-tools grub2"
+      }
+    }
+  },
   "family": {
     "gentoo": {
       "dkms_package": "",

diskimage_builder/elements/grub2/pkg-map

@@ -1,4 +1,13 @@
 {
+  "release": {
+    "rhel": {
+      "8": {
+        "signed_grub_efi": "efibootmgr grub2-efi-x64-modules grub2-efi-x64",
+        "signed_shim_efi": "shim",
+        "grub_bios": "grub2"
+      }
+    }
+  },
   "family":{
     "debian":{
       "signed_grub_efi": "grub-efi-amd64-signed",