build: set TARGET_ROOTFS_PARTSIZE to make combined image fit in 128MB

Change TARGET_ROOTFS_PARTSIZE from 128 to 104 MiB, so the whole image
(bootloader + boot + root) will fit on a 128MB CF card by default.

With these settings, the generated images (tested on x86-generic and
x86-64) have 126,353,408 bytes; the smallest CF card marketed as "128MB"
that I found a datasheet for (a Transcend TS128MCF80) has 126,959,616
bytes.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

config/Config-images.in

@@ -274,7 +274,7 @@ 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 104
 		help
 		  Select the root filesystem partition size.