ARCH is not available in .chroot hooks so we need to get the architecture manually.

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.525.14) UNRELEASED; urgency=medium
+
+  * More changes for raspi3 build support (LP: #1805668):
+    - Fix 100-purge-grub-legacy-ec2-arm.chroot to not gate on ARCH as that's
+      not defined in .chroot hooks.
+
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Fri, 18 Jan 2019 16:56:58 +0100
+
 livecd-rootfs (2.525.13) bionic; urgency=medium
 
   [ Balint Reczey ]

live-build/ubuntu-cpc/hooks/100-purge-grub-legacy-ec2-arm.chroot

@@ -1,7 +1,10 @@
 #!/bin/sh -eux
 
 # Only execute the hack for ARM images
-if [ "$ARCH" != "armhf" ] && [ "$ARCH" != "arm64" ]; then
+# ARCH is not available in .chroot hooks so we need to get the architecture
+# manually.
+arch=$(dpkg --print-architecture)
+if [ "$arch" != "armhf" ] && [ "$arch" != "arm64" ]; then
 	exit 0
 fi