Shrink rootfs

- With a 12G size passed to fallocate, we get a filesystem that actually
is longer than our total space according to the partition table, and
booting fails with this message:
[    2.691320] EXT4-fs (mmcblk0p2): bad geometry: block count 3145728 exceeds size of device (3133440 blocks)
- Set to 2G, if more space is needed rezise2fs can be used.

Change-Id: I145cc786d52f9ce7d1458ccf5c2a515649d08723
diff --git a/rootfs.mk b/rootfs.mk
index d6a6e73..f716af4 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -52,7 +52,7 @@
 	+make -f $(ROOTDIR)/build/debootstrap.mk validate-bootstrap-tarball
 
 	mkdir -p $(ROOTFS_DIR)
-	fallocate -l 12G $(ROOTFS_RAW_IMG)
+	fallocate -l 12834570240 $(ROOTFS_RAW_IMG)
 	mkfs.ext4 -j $(ROOTFS_RAW_IMG)
 	tune2fs -o discard $(ROOTFS_RAW_IMG)
 	sudo mount -o loop $(ROOTFS_RAW_IMG) $(ROOTFS_DIR)