live-build/auto/config: fail immediately on unknown arch/subarch.

When trying to make changes to refactor livecd-rootfs, it is difficult to
know what side effects a change may have because of the use of globs on
arch/subarch and the lack of an authoritative list of supported arch
combinations.

This assembles a list of all possible values for $ARCH:$SUBARCH by looking at
all existing uses of $SUBARCH in live-build/auto/config and combining with
etc/default-arches from ubuntu-cdimage:

$ grep + etc/default-arches |grep -vE '(trusty|xenial|bionic|focal)[[:space:]]|appliance'

It also includes a special-case *appliance* glob, because there are many
Ubuntu Core appliance builds and there may be more in the future, and we
don't want to have to update livecd-rootfs with a hard-coded list.

Otherwise, this includes all currently used / supported combinations.  The
amd64+kassel subarch is referenced in the code, but stopped being built a
while ago because "end of contract"; and there are some older no-longer-used
subarchs for particular raspi variants, which there is work to clean up
separately.  So this should be a no-op wrt buildability of any existing
images on mantic - and if not, that's important for us to know!

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (23.10.45) UNRELEASED; urgency=medium
+
+  * live-build/auto/config: fail immediately on unknown arch/subarch.
+
+ -- Steve Langasek <steve.langasek@ubuntu.com>  Fri, 22 Sep 2023 22:19:57 -0700
+
 livecd-rootfs (23.10.44) mantic; urgency=medium
 
   * Fix the subarch used in the hook to identify images for sifive

live-build/auto/config

@@ -1,6 +1,24 @@
 #! /bin/sh
 set -e
 
+case $ARCH:$SUBARCH in
+	amd64:|amd64:generic|amd64:intel-iot|\
+	arm64:|arm64:generic|arm64:raspi|arm64:snapdragon|\
+	arm64:tegra|arm64:tegra-igx|\
+	armhf:|armhf:raspi|\
+	i386:\
+	ppc64el:\
+	riscv64:icicle|riscv64:nezha|riscv64:licheerv|riscv64:unmatched|\
+	riscv64:visionfive|riscv64:visionfive2|\
+	s390x:\
+	*appliance*)
+		;;
+	*)
+		echo "Unknown architecture target $ARCH:$SUBARCH"
+		exit 1
+		;;
+esac
+
 if [ -e config/germinate-output ]; then
             mv -T config/germinate-output germ-tmp
             rm -rf config