I got interested in building Crochet images for Chromebook.  The first step, naturally is to build U-boot for Chromebook.

In order to build U-boot, you'll need to have already installed

• gmake
• gcc

You can get these from the ports tree.  You'll also need xdev, the FreeBSD cross-compiler toolkit.

I started with the latest U-boot 2014 source, which I got here.  The latest U-boot requires DTC 1.4.  I'm on FreeBSD 10, which has a much older DTC.

To build the latest DTC:

git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
cd dtc
gmake CC=gcc
cp dtc /usr/bin

Once we have DTC 1.4, we can build U-boot.

wget ftp://ftp.denx.de/pub/u-boot/u-boot-2014.07.tar.bz2
tar zxvf u-boot-2014.07.tar.bz2
cd u-boot-2014.07
gmake SED=gsed HOSTCC=cc CROSS_COMPILE=armv6-freebsd- snow_config
gmake SED=gsed HOSTCC=cc CROSS_COMPILE=armv6-freebsd-

The U-boot binary image is "u-boot.bin". To burn the U-boot binary to a SD card:

sudo dd if=u-boot.bin of=/dev/rdisk1

The next challenge is the file system. The file system on the Chromebook looks like this:

       start        size    part  contents
           0           1          PMBR (Boot GUID: E780FED6-7991-B94B-B622-FA7541BE9FBB)
           1           1          Pri GPT header
           2          32          Pri GPT table
     8671232    21295104       1  Label: "STATE"
                                  Type: Linux data
                                  UUID: D5488177-EB19-3243-8D7C-D6E4F01DB5B3
       20480       32768       2  Label: "KERN-A"
                                  Type: ChromeOS kernel
                                  UUID: 47715F23-7DD6-5545-93DC-3807D1CD4AFD
                                  Attr: priority=0 tries=0 successful=0
     4476928     4194304       3  Label: "ROOT-A"
                                  Type: ChromeOS rootfs
                                  UUID: BE1064FF-EC28-E64B-B3A1-A5C176D57C0F
       53248       32768       4  Label: "KERN-B"
                                  Type: ChromeOS kernel
                                  UUID: 5D200D96-1F52-804C-83A7-AD8081E8ACD4
                                  Attr: priority=1 tries=0 successful=1
      282624     4194304       5  Label: "ROOT-B"
                                  Type: ChromeOS rootfs
                                  UUID: D0795A2A-1D0D-4140-9F57-F87AEF399C44
       16448           1       6  Label: "KERN-C"
                                  Type: ChromeOS kernel
                                  UUID: E8A703B8-E0A5-964D-9508-D88DE336D003
                                  Attr: priority=0 tries=15 successful=0
       16449           1       7  Label: "ROOT-C"
                                  Type: ChromeOS rootfs
                                  UUID: 7A947ABA-E723-8440-9A07-0EC394C85195
       86016       32768       8  Label: "OEM"
                                  Type: Linux data
                                  UUID: 173B3C97-CDCF-F849-99E7-7614C6C0F9FD
       16450           1       9  Label: "reserved"
                                  Type: ChromeOS reserved
                                  UUID: CD83D43A-4FC8-4849-B0DD-368E9C3819BA
       16451           1      10  Label: "reserved"
                                  Type: ChromeOS reserved
                                  UUID: 344D3149-A5C2-B44B-A65C-2DFD59758FC5
          64       16384      11  Label: "RWFW"
                                  Type: ChromeOS firmware
                                  UUID: D9B79232-94AD-A143-9C49-F947BFA2491E
      249856       32768      12  Label: "EFI-SYSTEM"
                                  Type: EFI System Partition
                                  UUID: E780FED6-7991-B94B-B622-FA7541BE9FBB
    30777311          32          Sec GPT table
    30777343           1          Sec GPT header

There is a good description of the Chromium OS file system here.  The size parameter is in sectors, which are usually 512 bytes each.  So, for example the state partition is 10GB in size.  KERN-A is 16MB in size, and ROOT-A is 2GB.

The partition ids for ChromeOS kernel and rootfs, are "fe3a2a5d-4f32-41a7-b725-accc3285a309" and "3cb8e202-3b7e-47dd-8a3c-7ff2a13cfcec" respectively.  The partition ids for ChromeOS are documented here.