Samsung Chromebook (ARM)
The Samsung Chromebook (model XE303C12) is a laptop intended work in the cloud. It provides a powerful Cortex-A15 Dual Core Exynos 5 processor at 1.7 GHz with Mali-T604 GPU, 2 GiB of DDR3, 16 GiB of internal flash storage, WiFi a/b/g/n, SD and USB ports, HDMI connector and a 11.6" display. It is visually like the MacBook Air 11. With stock firmware it runs a heavily modified Gentoo-Linux, which means that nearly every code is available at the Google's git. It is possible to install ArchLinux ARM (ALARM) on this device.
This article is not meant to be an exhaustive setup guide and assumes that the reader has setup an Arch system before. Arch newbies are encouraged to read the Beginners' Guide if unsure how to preform standard tasks such as creating users, managing the system, etc.
Installing Arch Linux ARM
See the instructions at the archlinuxarm site. More information will be added here in the near future.
Flashing non-verified U-boot
The boot process of this Chromebook has several stages. The very first stages are burnt into the SoC (probably read-only although unconfirmed). Then, the SoC jumps to the starting of the 4 MiB SPI flash, which contains basically U-Boot.
This flash is split into a read-only half (first half) and read-write half. The read-only (RO) is loaded at the factory and contains several signature verifications (it is really a known-to-be-good U-Boot). The read-write (RW) is a signed U-Boot which also loads signed kernel and so on.
As you probably know (that's why you are reading this) you can load non-signed kernels by entering developer mode. But it has two drawbacks: you have to make a non-trivial process to install your bootloader and you have to hit Ctrl+D or Ctrl+U on every boot.
Removing RO protection
The good news is that you can disable the RO protection. The bad news is that you have to open your device. The recommended guide to open your Chromebook is the one at iFixit; stop at step 4, then jump to step 9. If you have a multimeter, ensure that after removal of the metallic sticker both parts of the ring are not in short-circuit.
Now you can write your U-Boot.
How to compile U-Boot
How to flash U-Boot
Booting from SD
If you ever screw things up by burning a wrong bootloader, there is a recovery mechanism. The SoC built-in bootloader (known as BL0 and BL1) can be configured to boot from SPI, USB, SD and probably other options. This is known because of the Arndale development board (and previous Exynos models behave the same as well).
This is configured at the factory to boot from SPI, but this is done with a bunch of resistors located at the bottom of the PCB (link to image). User (name) experimented and got the working configuration (link to image).
By short-circuiting the pads as per the diagram linked, your SoC will boot from SD. You have to (confirm this and complete the information) load a U-Boot with BL2 incorporated.