mosquitto_%.bbappend:

FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
SRC_URI += " file://mosquitto.conf "

$ ls -la mosquitto/
total 12
drwxrwxr-x 2 user user 4096 Jun  5 17:27 .
drwxrwxr-x 4 user user 4096 Jun  5 17:26 ..
-rw-r--r-- 1 user user    1 Jun  5 17:27 mosquitto.conf