Merge branch 'readme' into 'master'

Update board build instructions

See merge request Librem5/image-builder!60

README.md

 Image Builder
 =============
 
-Create a bootable SD Card image for the [Boundary Devices Nitrogen6max][1].
+This is the image builder for the Librem5 and related boards:
 
-These are WIP scripts tested on the Nit6QP_MAX.
+* *devkit*: The [Librem 5 Devkit][0]
+* *ec-som*: Emcraft's [i.MX 8M SOM Starter Kit][1]
+* *imx8*: [NXPs IMX8M Evluation Kit][2]
+* *imx6*: the Bounday Devices [Nit6QP_MAX][3]
 
-To build the image for then nitrogen6 run
+It supports various distributions (PureOS *labortory* and *purple*, Debian
+*buster*).  Additionally it can add our on commit built CI packages to get
+the latest versions of the Librem 5 related sofware.
 
-    make
+Furthermore it can incoproate different kernel versions:
 
-The resulting image will be named *imx6.img*.
+* *latest*: Fetch the most recently built kernel
+* *ci*: deprecated, don't use
+* *unstable*: Pre release kernels (that got some testing)
+* *stable*: Released kernels (none of them exist yet)
 
-To build an image for the imx8 EVK
+So to build an image with all the latest software you'd use:
 
-    make BOARD=imx8
-
-The resulting image will be named *imx8.img*.
+    ./build-image -T latest -b devkit -d buster+ci
 
 # 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
@@ -37,4 +46,7 @@ You can run tests against an installed board using
     cd tests
     TARGET_HOST=<your_board_ip> rake spec:nitrogen6
 
-[1]: https://boundarydevices.com/product/nitrogen6max/
+[0]: https://developer.puri.sm/Librem5/Hardware_Reference.html#librem-5-devkit
+[1]: https://www.emcraft.com/products/868
+[2]: https://www.nxp.com/support/developer-resources/run-time-software/i.mx-developer-resources/evaluation-kit-for-the-i.mx-8m-applications-processor:MCIMX8M-EVK
+[3]: https://boundarydevices.com/product/nitrogen6max/