vsftpd: enable user authentication through PAM when it is enabled

https://jira.digi.com/browse/DEL-6502

Signed-off-by: Arturo Buzarra <arturo.buzarra@digi.com>

meta-digi-dey/recipes-connectivity/vsftpd/vsftpd_%.bbappend

@@ -3,6 +3,10 @@
 FILESEXTRAPATHS_prepend := "${THISDIR}/${BPN}:"
 
 do_install_append() {
+    if ! test -z "${PAMLIB}" ; then
+        # Access through Pluggable Authentication Modules (PAM)
+        echo "pam_service_name=vsftpd" >> ${D}${sysconfdir}/vsftpd.conf
+    fi
     if ${@bb.utils.contains('DISTRO_FEATURES', 'systemd', 'true', 'false', d)}; then
         install -d ${D}${sysconfdir}/tmpfiles.d
         echo "d /run/vsftpd/empty 0755 root root -" \