Change volatile log control to image feature 'volatile-log'

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>

conf/distro/include/mortsgna-image-logfile-volatile.inc

@@ -1,11 +1,13 @@
 # override bitbake.conf default
 VOLATILE_LOG_DIR = "false"
 
+IMAGE_FEATURES[validitems] += "volatile-log"
+
 # Do alignment on journal and /var/log
 ROOTFS_POSTPROCESS_COMMAND += "postinst_volatile_log; "
 
 postinst_volatile_log () {
-    if [ "${MORTSGNA_IMAGE_VOLATILE_LOG_DIR}" = "1" ]; then
+    if [ "${@bb.utils.contains('IMAGE_FEATURES', 'volatile-log', '1', '0', d)}" = "1" ]; then
         # systemd journal
         if [ -f "${IMAGE_ROOTFS}${sysconfdir}/systemd/journald.conf" ]; then
             sed -i \