ar71xx: image: add mtdpartsize macro

The new macro will be used to determine the firmware
partition size from an MTD layout.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38117

target/linux/ar71xx/image/Makefile

@@ -31,6 +31,10 @@ define mkcmdline
 $(if $(1),board=$(1) )$(if $(2),console=$(2)$(COMMA)$(3))
 endef
 
+define mtdpartsize
+$(shell echo $$((`echo '$(2)' | sed -e 's/.*[:$(COMMA)]\([0-9]*\)k[@]*[0-9a-zx]*($(1)).*/\1/'` * 1024)))
+endef
+
 SINGLE_PROFILES:=
 
 define SingleProfile