mediatek: mt7623: allow build without ext4 rootfs

The image generation code for the U7623 board expects ext4 filesystem
to be selected in menuconfig and CONFIG_TARGET_ROOTFS_PARTSIZE to be
defined. Now that ext4 isn't enabled any more, the variable was missing
and broke the build.
Set the default (104) instead of using the config variable to fix that.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/image/mt7623.mk

@@ -132,7 +132,7 @@ define Build/mtk-mmc-img
 		$@.boot \
 		$(IMAGE_ROOTFS) \
 		$(CONFIG_MTK_BOOT_PARTSIZE) \
-		$(CONFIG_TARGET_ROOTFS_PARTSIZE)
+		104
 endef
 
 define Build/scatterfile