sdcard_image bbclass: move back to 2GB images, clarify LOOPDEV* usage

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>

classes/sdcard_image.bbclass

@@ -7,6 +7,7 @@ IMAGE_FSTYPES_append = " tar.bz2 sdimg"
 LOSETUP ?= "/sbin/losetup"
 
 # Since these need to go in /etc/fstab we can hardcode them
+# Since the vars are weakly assigned, you can override them from your local.conf
 LOOPDEV ?= "/dev/loop1"
 LOOPDEV_BOOT ?= "/dev/loop2"
 LOOPDEV_FS ?= "/dev/loop3"
@@ -28,7 +29,7 @@ IMAGE_CMD_sdimg () {
 
 	# If an SD image is already present, reuse and reformat it
 	if [ ! -e ${SDIMG} ] ; then
-		dd if=/dev/zero of=${SDIMG} bs=$(echo '255 * 63 * 512' | bc) count=444
+		dd if=/dev/zero of=${SDIMG} bs=$(echo '255 * 63 * 512' | bc) count=222
 	fi
 
 	${LOSETUP} ${LOOPDEV} ${SDIMG}