Flashing your own Coreboot-version and Payload onto your device is do-able, but requires some hardware preperation and ignorance in warning messages.
A good place to start is the Chromium site. They have guides howto set your Chromebook into developer-mode and howto disassemble your device.
Both things will be nessary, if you want to flash your own version of Coreboot onto your Chromebook (in-system). It is nessary to close a circuit on the motherboard via either a switch, jumper or srew to disable the write-protection of the spi chip. And to use the flashing tool flashrom in ChromeOs, you have to be in the developer-mode. If you already installed a Gnu/Linux-system or likely you can also use the flashrom from there (but then you would be in developer-mode anyway).
Note: The first three Chromebooks are not supported by coreboot at the moment, but flashrom should be working. Mario, Alex and ZGB are running Insyde H2C some UEFI. Snow has das U-Boot nativly installed, but a coreboot port is available. All the others come shipped with Coreboot.
|Release date||Manufacturer||Model||Project Code Name||Close cicuit via||Location for Jumper/Screw||SPI-Chip|
|May 2012||Samsung||Series 3 Chromebox||Stumpy||Jumper||between the Battery and Ram|
|May 2012||Samsung Series 5||550 Chromebook||Lumpy||Jumper||between the Battery and Ram|
|October 2012||Samsung||ARM Chromebook||Snow (aka Daisy)||Screw||next to usb3.0-port|
|November 2012||Acer||C7 Chromebook||Parrot||Jumper||between CPU and Fan under plastic|
|January 2013||Lenovo||Thinkpad X131e Chromebook||Stout||?|
|February 2013||HP||Pavilion 14 Chromebook||Butterfly||Switch||rightside behind the usb-ports|
|February 2013||Chromebook Pixel||Link||Screw|
General Hardware Preperation
Use the disassembly guides to disassemble your Chromebook till you see the place were you can put on a jumper, screw in a bolt or flip a switch. Check the out the pictures in the disassembly guides to find the right spot. Either screw, switch or plug on now. For example in Lumpy and Stumpy you have to put a jumper on place between the Battery and Ram next to the Keyboard-Connector. Now assemble again (follow the guide). Now you are ready to flash your own Coreboot-version onto your Chromebook.
Building your own Coreboot-version and Payload
First, you cannot do that inside the ChromeOS. Use your GNU/Linux-system and follow the Build HOWTO.
Ok, you fucked things up, but on the other hand this will give you the oppotunity to learn external chip-programming, so cheer-up. First you will need to locate the SPI-Chip. If you done that you will need an external flashrom programmer (for example the open-hardware tool Bus Pirate). At the moment ask at the coreboot/flashrom mailing list for further details.