proftpd: use <pkg>_INSTALL_INIT_SYSV mechanism

Also use a complete destination path for the $(INSTALL) command.

[Peter: use install -D as noted by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/proftpd/proftpd.mk

@@ -52,7 +52,10 @@ define PROFTPD_INSTALL_TARGET_CMDS
 		$(INSTALL) -m 0644 -D $(@D)/sample-configurations/basic.conf $(TARGET_DIR)/etc/proftpd.conf; \
 		$(if $(BR2_INET_IPV6),,$(SED) 's/^UseIPv6/# UseIPv6/' $(TARGET_DIR)/etc/proftpd.conf;) \
 	fi
-	$(INSTALL) -m 0755 package/proftpd/S50proftpd $(TARGET_DIR)/etc/init.d
+endef
+
+define PROFTPD_INSTALL_INIT_SYSV
+	$(INSTALL) -D -m 0755 package/proftpd/S50proftpd $(TARGET_DIR)/etc/init.d/S50proftpd
 endef
 
 $(eval $(autotools-package))