qualcommax: make header length configurable on Build/mstc-header

I-O DATA WN-DAX3000GR has a MSTC (Mitra Star Technology Corp.) specific
header with a different length than ELECOM WRC-X3000GS2.
Make the header length configurable by parameterizing on
Build/mstc-header.

- WRC-X3000GS2: 0x400
- WN-DAX3000GR: 0x480

Signed-off-by: INAGAKI Hiroshi <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19053
Signed-off-by: Robert Marko <[email protected]>

target/linux/qualcommax/image/ipq50xx.mk

@@ -1,7 +1,8 @@
 DEVICE_VARS += BOOT_SCRIPT
 
 define Build/mstc-header
-	$(eval version=$(1))
+	$(eval version=$(word 1,$(1)))
+	$(eval hdrlen=$(if $(word 2,$(1)),$(word 2,$(1)),0x400))
 	gzip -c $@ | tail -c8 > [email protected]
 	( \
 		printf "CMOC"; \
@@ -10,7 +11,7 @@ define Build/mstc-header
 			dd bs=64 count=1 conv=sync 2>/dev/null; \
 		printf "$(version)" | \
 			dd bs=64 count=1 conv=sync 2>/dev/null; \
-		dd if=/dev/zero bs=884 count=1 2>/dev/null; \
+		dd if=/dev/zero bs=$$(($(hdrlen) - 0x8c)) count=1 2>/dev/null; \
 		cat $@; \
 	) > [email protected]
 	mv [email protected] $@