Refresh the BeagleBone Black image

Barebox won't build (it complains about <linux/list.h> not found and I
cannot easily solve that), so let's go back to U-Boot.

The 2019.01 U-Boot release is buggy in its recognizing of partition
tables (`fatls` complains, for example), and the whole thing reverts to
a "legacy boot", so let's use the latest release, the 2019.07.

I'm also giving up on the g_cdc module because the board that I'm using
has a microUSB header for the debug console.

- FIXME: Properly distinguish between an SD-card boot and an eMMC boot
(right now it is hardcoded to boot from the SD card)
- FIXME: test Rauc updates once eMMC works
- FIXME: activate watchdog from U-Boot

Change-Id: I013f145cf2fe24e41ea0dafd225e3feb1854712c
diff --git a/board/czechlight/beaglebone/uboot.fragment b/board/czechlight/beaglebone/uboot.fragment
new file mode 100644
index 0000000..d231b5d
--- /dev/null
+++ b/board/czechlight/beaglebone/uboot.fragment
@@ -0,0 +1 @@
+CONFIG_CMD_SETEXPR=y