mosquitto: optionally include mosquitto_passwd utility

Many users of the SSL build of mosquitto need the passwd utility for
managing keys.

Fixes github issue #1909

Signed-off-by: Karl Palsson <karlp@remake.is>

net/mosquitto/Config.in

     default y
     help
         Includes websockets support in the broker, via libwebsockets
+
+config MOSQUITTO_PASSWD
+    bool "Include mosquitto_passwd utility"
+    depends on PACKAGE_mosquitto
+    default y
+    help
+        mosquitto_passwd is a tool for managing password files for mosquitto.

net/mosquitto/Makefile

 define Package/$(PKG_NAME)/install
 	$(INSTALL_DIR) $(1)/usr/sbin
 	$(INSTALL_BIN) $(PKG_BUILD_DIR)/src/mosquitto $(1)/usr/sbin/mosquitto
+ifeq ($(CONFIG_MOSQUITTO_PASSWD),y)
+        $(INSTALL_BIN) $(PKG_BUILD_DIR)/src/mosquitto_passwd $(1)/usr/sbin
+endif
 	$(INSTALL_DIR) $(1)/etc/mosquitto
 	$(INSTALL_CONF) $(PKG_BUILD_DIR)/mosquitto.conf $(1)/etc/mosquitto/mosquitto.conf
 	$(INSTALL_DIR) $(1)/etc/init.d