beaglebone: adds barebox bootloader

jkt: This is a port of https://patchwork.ozlabs.org/patch/614226/

Change-Id: I646d843fe2b9248d5a2ed3855142f1545b7bab52
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
diff --git a/board/czechlight_beaglebone/barebox.env/config-board b/board/czechlight_beaglebone/barebox.env/config-board
new file mode 100644
index 0000000..cd7b26d
--- /dev/null
+++ b/board/czechlight_beaglebone/barebox.env/config-board
@@ -0,0 +1,4 @@
+#!/bin/sh
+
+global.boot.default=sd
+