board/olpc: add readme.txt

This adds some instructions on how to build and run images for the OLPC
XO laptops.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

diff --git a/board/olpc/readme.txt b/board/olpc/readme.txt
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+OLPC XO Laptops
+===============
+
+This document explains how to build and run images that run on the OLPC
+XO laptops.
+
+Supported models
+----------------
+
+* OLPC XO-1
+  The original NS Geode based OLPC laptop, uses the x86 architecture.
+
+* OLPC XO-7.5
+  The ARM-based laptop. Needs a recent enough firmware to provide a good
+  enough flattened device tree to the kernel.
+
+Configure and build
+===================
+
+  $ make olpc_xo1_defconfig   # Configure for XO-1
+
+or:
+
+  $ make olpc_xo175_defconfig # Configure for XO-1.75
+
+Then:
+
+  $ make menuconfig           # Customize the build configuration
+  $ make                      # Build
+
+Create the bootable media
+=========================
+
+When the build is finished, the resulting image file will be called
+"sdcard.img". It can be written directly to a SD-card or and USB flash stick.
+
+Please double check that you're using the right device (e.g. with "lsblk"
+command). Doing the following will DESTROY ALL DATA that's currently on the
+media.
+
+  # cat output/images/sdcard.img >/dev/<device>
+
+Preparing the machine
+=====================
+
+Firmware security
+-----------------
+
+Most OLPC machines were shipped with the security system that disallows
+booting unsigned software. If this is the case with your machine, in order
+to run the image you've built on it you'll need to get a developer key and
+deactivate the security system.
+
+The procedure is descriped in the OLPC wiki:
+http://wiki.laptop.org/go/Activation_and_Developer_Keys
+
+Firmware upgrade
+----------------
+
+It is always preferrable to use an up to date firmware. The firmware images
+are available at http://wiki.laptop.org/go/Firmware. For the XO-1.75 laptop
+to boot the mainline kernel a firmware Q4E00JA or newer is needed. You can
+get it at http://dev.laptop.org/~quozl/q4e00ja.rom.
+
+To update the firmware, place the .rom file on to your bootable media,
+connect a charged battery pack and a wall adapter, and enter the Open
+Firmware prompt by pressing ESC during the early boot (needs an unlocked
+laptop -- see "Firmware security" above). Then use the "flash" command
+to update the firmware:
+
+  ok flash ext:\q4e00ja.rom   \ Flash the "q4e00ja.rom" from the SD card
+  ok flash u:\q4e00ja.rom     \ Flash the "q4e00ja.rom" from USB stick
+
+Booting the machine
+===================
+
+Once your machine is unlocked, it will automatically boot from your media
+wherever it will detect it attached to the USB bus or the SD card slot.