Fix grub-efi-amd64-signed install failure

When build ubuntu iso image, it will install grub-efi-amd64-signed
and grub-efi-amd64 packages. Both of the postinst script will try
to find root device and install grub which will definitely fail in
such a chroot environment.
So the workaround is to skip error and remove postinst script.
And confirm the package be installed successfully at last.

Change-Id: Ie0aecb212b22362046db55b5ad8c64c3211c28e5
Closes-Bug: #1491280
Co-Authored-By: Jane.zhang <jian.zhang8@hpe.com>

elements/grub2/install.d/02-grub-install-spec

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-1} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+# grub-efi-amd64-signed in ubuntu post will try to call grub-efi-amd64 to install grub into root
+# device which definitely fail at this stage
+# the workaround is to skip error and remove postinst script
+if [[ "ubuntu" =~ "$DISTRO_NAME" ]]; then
+    if ! install-packages grub-efi-amd64-signed; then
+        rm -f /var/lib/dpkg/info/grub-efi-amd64-signed.postinst
+        rm -f /var/lib/dpkg/info/grub-efi-amd64.postinst
+    fi
+    dpkg --configure grub-efi-amd64
+    dpkg --configure grub-efi-amd64-signed
+fi

elements/grub2/pkg-map

@@ -1,4 +1,11 @@
 {
+  "distro":{
+    "ubuntu":{
+      "signed_grub_efi": "",
+      "signed_shim_efi": "shim-signed",
+      "grub_bios": "grub-pc-bin"
+    }
+  },
   "family":{
     "debian":{
       "signed_grub_efi": "grub-efi-amd64-signed",