Fix image builds for the VFAT USB flash

It's hard to fit 360+MB worth of data into a 350MB disk image.

The math is actually not as simple as taking a look at the rootfs sizes
or the on-disk consumption of the target/ directory because what ends up
on the USB disk image is the compressed CPIO root and a squashfs image.
Anyway, allow some breathing space here.

Change-Id: Ia54a9ae640b4f80a6311b9c234e131dfdb8d31e4
diff --git a/board/czechlight/clearfog/usb-genimage.cfg b/board/czechlight/clearfog/usb-genimage.cfg
index 2260dd8..bd2ed6e 100644
--- a/board/czechlight/clearfog/usb-genimage.cfg
+++ b/board/czechlight/clearfog/usb-genimage.cfg
@@ -10,7 +10,7 @@
 			"usb-reflash-factory.sh"
 		}
 	}
-	size = 350M
+	size = 400M
 }
 
 image usb-flash.img {