bootstrap_cache: fix failure condition

during testing i encountered an unexpected error resulting from the
following condition:
 - bootstrap was cached
 - cache of bootstrap packages was empty (from playing with `lb clean`)
 - installer was not none|live

everything works fine up until the main installer script, which comes to
an abrupt halt with an error due to the missing cached bootstrap packages
that it wants to copy.

this situation is easy to unintentionally create, as i managed to do.

here we catch the failure condition, correcting for it.

this is done by checking for the missing cached packages when restoring
the bootstrap from the cache, and skipping this if the packages were
missing, thus forcing the bootstrap to be rebuilt. the packages should
then be found within the cache, allowing the installer stage to complete
successfully.

of course the bootstrap stage will only cache the packages if caching is
enabled, but if caching is disabled and installer enabled, the config
validation will catch that, reporting that problem.

Gbp-Dch: Short

scripts/build/bootstrap_cache

@@ -55,6 +55,20 @@ case "${_ACTION}" in
 			# Removing old chroot
 			rm -rf chroot
 
+			# Catch failure condition.
+			# The installer stage, if not 'none' or 'live' needs to copy cached
+			# bootstrap packages, and will error if these do not exist. This
+			# could be the case if bootstrap is cached but the packages were
+			# wiped from the cache. This detects this condition and if it is the
+			# case, ignores the cached bootstrap, forcing it to be rebuilt thus
+			# re-caching the packages.
+			if ! In_list "${LB_DEBIAN_INSTALLER}" live none &&
+			   ! ls cache/packages.bootstrap/*.deb > /dev/null 2>&1
+			then
+				Echo_message "Cached bootstrap packages required but missing, ignoring cached bootstrap to force rebuild."
+				exit 0
+			fi
+
 			# Restoring old cache
 			cp -a cache/bootstrap chroot