ar71xx: Fix build for dap-1330-a1 board

Using a version number of 16 character causes a buffer overflow in the
version number overwriting the first bit of the signature in the
mkdapimg2 tool.
I am not sure if the version number should be null terminated or not.
This patch reduces the size of the version number by removing the number
of private commits from it.

This was the original version number which caused problems:
OpenWrt-r6727+10
Now it uses this version number:
OpenWrt-r6727

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ar71xx/image/generic.mk

@@ -36,7 +36,7 @@ define Build/mkdapimg2
 	$(STAGING_DIR_HOST)/bin/mkdapimg2 \
 		-i $@ -o $@.new \
 		-s $(DAP_SIGNATURE) \
-		-v $(VERSION_DIST)-$(firstword $(subst -, ,$(REVISION))) \
+		-v $(VERSION_DIST)-$(firstword $(subst +, ,$(firstword $(subst -, ,$(REVISION))))) \
 		-r Default \
 		$(if $(1),-k $(1))
 	mv $@.new $@