README.md : add documentaion for build_uboot.sh

README.md

@@ -23,6 +23,21 @@ So to build an image with all the latest software you'd use:
 
     ./build-image -T latest -b devkit -d buster+ci
 
+# Build u-boot
+
+The build_uboot.sh script can build u-boots for a number of targets
+
+* *devkit-recovery*: This is to boot the [Librem 5 Devkit][0] via SDP
+* *devkit*: The [Librem 5 Devkit][0]
+* *ec-som*: Emcraft's [i.MX 8M SOM Starter Kit][1]
+* *imx8*: [NXPs IMX8M Evaluation Kit][2]
+* *imx6*: The Bounday Devices [Nit6QP_MAX][3]
+
+From the build directory invoke build_uboot.sh
+
+    cd build 
+    ./build_uboot.sh -b devkit
+
 # Creating a bootable SD card
 
 This is only relevant for SD card booting on e.g. the Nitrogen or NXP