README: drop uboot explanation

 We build it via a separate repo

README.md

@@ -11,42 +11,12 @@ It supports various distributions (PureOS *byzantium* and *amber-phone*.
 
 So to build an image with all the latest software you'd use:
 
-    ./build-image -T latest -b devkit -d buster+ci
+    ./build-image -b librem5r4 -d byzantium
 
 or just issue
 
     make
 
-# 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
-boards:
-
-The SD card must be larger than 3G. Replace <x> with the SD card device.
-
-    sudo dd if=imx6.img of=/dev/sd<x> bs=4M
-
-or a bit more verbose and using the compressed image
-
-    bin/write-image of=/dev/sd<x>
-
-Note: All data on the SD card will be overwritten.
-
 # Running the tests
 
 You can run tests against an installed board using