Support booting a uImage

Meta:
Headline: TBD
Acceptance: TBD

Implement new action somewhat like:

{
    "action": "uboot",
    "paramters": {
        "file_urls": ["http://people.linaro.org/~l33t-hacker/uImage"],
        "boot_script": ["tftp uImage", "bootm"],
        "success_pattern": "\(initrd\)#",
        "timeout": 300
    }
}

This would download the files referenced by file_urls, make them available via tftp, boot into uboot, set up the tftp parameters in uboot so that they can be downloaded, execute the commands from boot_script, wait for success_pattern and report success if it appears before timeout seconds elapse.

Setting up uboot involves:

1. Giving the board an ip address (setenv ipaddress or something)
2. Setting the address of the tftp server (setenv serverip)
3. Set the port of the tftp server (setenv tftpdstport)

My idea is to run a separate (probably Python) tftp server on a random (bind(('', 0))) port and then tell the board to pull files from there.

Implementation-wise this can probably be done by adding a "to_bootloader" argument to power_on().

Work items:
find tftp implementation: TODO
implement making files available to tftp: TODO
implement rest of action: TODO