clearfog: USB autoboot on eMMC bring-up

Let's use U-Boot's distro boot configuration for auto-executing the boot
script. This requires a proper name of the file and also a partition
that is marked as bootable.

This only helps on a system that is already jumpered to booting from
UART and where the MMC memory is unpartitioned. If we're recovering from
a broken eMMC flash, this won't really help.

This setup is also not enough to auto-boot from a factory-installed
Marvell-specific U-Boot. Even with the flash disk plugged in, it fails
with a "Bad Linux ARM zImage magic!". We will therefore have to keep
booting from UART for now...

Change-Id: I25caf68e33c47ab326b6c87d3080660575d7ff10
diff --git a/board/czechlight/clearfog/genimage.cfg b/board/czechlight/clearfog/genimage.cfg
index 01866bb..548f538 100644
--- a/board/czechlight/clearfog/genimage.cfg
+++ b/board/czechlight/clearfog/genimage.cfg
@@ -39,8 +39,8 @@
 
 image usb-flash.vfat {
 	vfat {
+		file "boot.scr" { image = "usb-boot.scr" }
 		files = {
-			"usb-boot.scr",
 			"zImage",
 			"czechlight-clearfog.dtb",
 			"rootfs.cpio.uboot"
@@ -55,6 +55,7 @@
 
 	partition boot {
 		partition-type = 0xc
+		bootable = true
 		image = "usb-flash.vfat"
 	}
 }