build: make TARGET_ROOTFS_PARTSIZE 128MiB by default

As we're now going to pad all images by default, lets decrease the
default rootfs partition size from 256MiB to 128MiB in order to save
some space.

I'm keeping it above 100MiB in order to keep current behavior, where
overlay filesystem is using F2FS.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

config/Config-images.in

@@ -268,7 +268,7 @@ menu "Target Images"
 	config TARGET_ROOTFS_PARTSIZE
 		int "Root filesystem partition size (in MB)"
 		depends on GRUB_IMAGES || USES_ROOTFS_PART || TARGET_ROOTFS_EXT4FS || TARGET_omap || TARGET_rb532 || TARGET_sunxi || TARGET_uml
-		default 256
+		default 128
 		help
 		  Select the root filesystem partition size.