ramips: sercomm-payload: use pide-file argument

Instead of passing an array of hex bytes for the Sercomm PID we can now use
the --pid-file parameter.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

target/linux/ramips/image/common-sercomm.mk

@@ -51,7 +51,7 @@ define Build/sercomm-payload
 	$(TOPDIR)/scripts/sercomm-payload.py \
 		--input-file $@ \
 		--output-file $@.tmp \
-		--pid "$$(cat $@.pid | od -t x1 -An -v | tr -d '\n')"
+		--pid-file $@.pid
 	mv $@.tmp $@
 	rm $@.pid
 endef