bootloader: fix arm64 install path

This fixes a regression introduced by
Ia99687815667c3cf5e82cf21d841d3b1008b8fa9

It turns out that [[ -d /usr/lib/grub/*-efi ]] is not a good check,
because [[ doesn't split that and try to glob match ( [ would ).  This
has resulted in us triggering this path on ARM64.

This is an x86-64 only check, because on other platforms we either
don't support EFI or are EFI only.  Restrict this check to get arm64
working again.

Change-Id: I6a75f8504826bcb0ac122d53dfb9faff975077f4

diskimage_builder/elements/bootloader/finalise.d/50-bootloader

@@ -94,10 +94,13 @@ GRUB_OPTS="--force "
 # * --target tells grub what's the target platform
 # * the boot images are placed in /usr/lib/grub/<cpu>-<platform>
 # * i386-pc is used for BIOS-based machines
-# http://www.gnu.org/software/grub/manual/grub.html#Installation
-#
-if [[ -d /sys/firmware/efi && ! -d /usr/lib/grub/*-efi ]]; then
-    GRUB_OPTS="$GRUB_OPTS --target=i386-pc"
+#   http://www.gnu.org/software/grub/manual/grub.html#Installation
+# * this check is only valid for x86_64; other platforms we support
+#   are either not EFI (ppc) or always EFI (arm64)
+if [[ "x86_64 amd64" =~ ${ARCH} ]]; then
+    if [[ -d /sys/firmware/efi && ! -d /usr/lib/grub/x86_64-efi ]]; then
+        GRUB_OPTS="$GRUB_OPTS --target=i386-pc"
+    fi
 fi
 
 if [[ "$ARCH" =~ "ppc" ]] ; then