base-files: use JSON for storing firmware validation info

So far firmware validation result was binary limited: it was either
successful or not. That meant various limitations, e.g.:
1) Lack of proper feedback on validation problems
2) No way of marking firmware as totally broken (impossible to install)

This change introduces JSON for storing detailed validation info. It
provides a list of performed validation tests and their results. It
allows marking firmware as non-forceable (broken image that can't be
even forced to install).
Example:
{
        "tests": {
                "fwtool_signature": true,
                "fwtool_device_match": true
        },
        "valid": true,
        "forceable": true
}

Implementation is based on *internal* check_image bash script that:
1) Uses existing validation functions
2) Provides helpers for setting extra validation info

This allows e.g. platform_check_image() to call notify_check_broken()
when needed & prevent user from bricking a device.

Right now the new JSON info is used by /sbin/sysupgrade only. It still
doesn't make use of "forceable" as that is planned for later
development.

Further plans for this feature are:
1) Expose firmware validation using some new ubus method
2) Move validation step from /sbin/sysupgrade into "sysupgrade" ubus
   method so:
   a) It's possible to safely sysupgrade using ubus only
   b) /sbin/sysupgrade can be more like just a CLI

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit f522047958)

package/base-files/files/sbin/sysupgrade

@@ -2,6 +2,7 @@
 
 . /lib/functions.sh
 . /lib/functions/system.sh
+. /usr/share/libubox/jshn.sh
 
 # initialize defaults
 export MTD_ARGS=""
@@ -191,9 +192,6 @@ add_overlayfiles() {
 	return 0
 }
 
-# hooks
-sysupgrade_image_check="fwtool_check_signature fwtool_check_image platform_check_image"
-
 if [ $SAVE_OVERLAY = 1 ]; then
 	[ ! -d /overlay/upper/etc ] && {
 		echo "Cannot find '/overlay/upper/etc', required for '-c'" >&2
@@ -316,17 +314,19 @@ case "$IMAGE" in
 		;;
 esac
 
-for check in $sysupgrade_image_check; do
-	( $check "$IMAGE" ) || {
-		if [ $FORCE -eq 1 ]; then
-			echo "Image check '$check' failed but --force given - will update anyway!" >&2
-			break
-		else
-			echo "Image check '$check' failed." >&2
-			exit 1
-		fi
-	}
-done
+json_load "$(/usr/libexec/validate_firmware_image "$IMAGE")" || {
+	echo "Failed to check image"
+	exit 1
+}
+json_get_var valid "valid"
+[ "$valid" -eq 0 ] && {
+	if [ $FORCE -eq 1 ]; then
+		echo "Image check failed but --force given - will update anyway!" >&2
+	else
+		echo "Image check failed." >&2
+		exit 1
+	fi
+}
 
 if [ -n "$CONF_IMAGE" ]; then
 	case "$(get_magic_word $CONF_IMAGE cat)" in