Merge from bittboy/buildroot@26c91a9

board/chromebook/elm/readme.txt

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+Mediatek MT8173 aka Chromebook Elm
+==================================
+
+This file describes booting the Chromebook from an SD card containing
+Buildroot kernel and rootfs, using the original bootloader. This is
+the least invasive way to get Buildroot onto the devices and a good
+starting point.
+
+The bootloader will only boot a kernel from a GPT partition marked
+bootable with cgpt tool from vboot-utils package.
+The kernel image must be signed using futility from the same package.
+The signing part is done by sign.sh script in this directory.
+
+It does not really matter where rootfs is as long as the kernel is able
+to find it, but this particular configuration assumes the kernel is on
+partition 1 and rootfs is on partition 2 of the SD card.
+Make sure to check kernel.args if you change this.
+
+Making the boot media
+---------------------
+Start by configuring and building the images.
+
+	make chromebook_elm_defconfig
+	make menuconfig # if necessary
+	make
+
+The important files are:
+
+	uImage.kpart (kernel and device tree, signed)
+	rootfs.tar
+	bootsd.img (SD card image containing both kernel and rootfs)
+
+Write the image directly to some SD card.
+WARNING: make sure there is nothing important on that card,
+and double-check the device name!
+
+	SD=/dev/mmcblk1		# may be /dev/sdX on some hosts
+	dd if=output/images/bootsd.img of=$SD
+
+Switching to developer mode and booting from SD
+-----------------------------------------------
+Power Chromebook down, then power it up while holding Esc+F3.
+BEWARE: switching to developer mode deletes all user data.
+Create backups if you need them.
+
+While in developer mode, Chromebook will boot into a white screen saying
+"OS verification is off".
+
+Press Ctrl-D at this screen to boot Chromium OS from eMMC.
+Press Ctrl-U at this screen to boot from SD (or USB)
+Press Power to power it off.
+Do NOT press Space unless you mean it.
+This will switch it back to normal mode.
+
+The is no way to get rid of the white screen without re-flashing the bootloader.
+