hostapp-update-hooks: add debug mode

This provides an easy switch to enable tracing on HUP hooks that works
both on old and new OS hooks as enabling it depends on a config.json
setting.

It is meant to debug field issues with HUP failure where all we see is:
```
Before hooks (old os) ran successfully
Failed to run the new hooks. Running current hooks..
```
For example:
https://jel.ly.fish/support-thread-1-0-0-front-cnv-eq6ipvx

Change-type: patch
Signed-off-by: Alex Gonzalez <alexg@balena.io>

meta-balena-common/recipes-support/hostapp-update-hooks/files/hostapp-update-hooks

@@ -4,6 +4,8 @@ set -o errexit
 
 # shellcheck disable=SC1091
 . /usr/libexec/os-helpers-logging
+# shellcheck disable=SC1091
+. /usr/sbin/balena-config-defaults
 
 old_os_before_hooks=0
 old_os_after_hooks=0
@@ -40,6 +42,15 @@ Options:
 EOF
 }
 
+DEBUG=$(jq -r '.debug // empty' "${CONFIG_PATH}")
+run_hook () {
+	if [ "${DEBUG}" = "1" ]; then
+		/bin/sh -x "$1"
+	else
+		"$1"
+	fi
+}
+
 # Parse arguments
 while [ "$#" -gt "0" ]; do
 	key=$1
@@ -85,13 +96,13 @@ list_of_old_os_after_hooks=$(find "${HOOKS_DIR}" -type f | grep after | sort)
 
 if [ "$old_os_before_hooks" = "1" ] ; then
 	for hook in $list_of_old_os_before_hooks; do
-		"$hook"
+		run_hook "$hook"
 	done
 fi
 
 if [ "$forward_hooks" = "1" ] ; then
 	for hook in $list_of_forward_hooks; do
-		if ! "$hook" ; then
+		if ! run_hook "$hook" ; then
 			forward_cleanup=1
 			break
 		fi
@@ -100,19 +111,19 @@ fi
 
 if [ "$old_os_after_hooks" = "1" ] ; then
 	for hook in $list_of_old_os_after_hooks; do
-		"$hook"
+		run_hook "$hook"
 	done
 fi
 
 if [ "$commit_hooks" = "1" ] ; then
 	for hook in $list_of_forward_commit_hooks; do
-		"$hook"
+		run_hook "$hook"
 	done
 fi
 
 if [ "$forward_cleanup" = "1" ] ; then
 	for hook in $list_of_forward_cleanup_hooks; do
-		"$hook"
+		run_hook "$hook"
 	done
 	exit 1
 fi