Improvements for bootloader testing

• Allow extra files to be deployed to the TFTP server, such as a test scripts '.itb' file.
• Allow the TFTP deployment to be used without a kernel image, for test cases where we don't actually boot Linux.
• Tidy up the docs a little while making these changes.

As an example of how we're using this, see the following actions list. The tests.itb file contains a couple of test scripts which can be executed in u-boot using the source command. This image is deployed to the TFTP server and then downloaded to the DUT as part of an "interactive" test.

- deploy:
    to: tftp
    extra_files:
      tests:
        url: https://example.com/raw/tests/tests.itb

- boot:
    method: bootloader
    bootloader: u-boot
    commands: []
    prompts:
    - "=>"

- test:
    timeout:
      minutes: 20
    interactive:
    - name: test_script
      prompts: ["\n=> "]
      script:
      # Download and run test scripts
      - command: "setenv autoload no && dhcp"
        name: dhcp
        successes:
        - message: "DHCP client bound to address"

      - command: "setenv serverip {SERVER_IP} && tftp 0x48000000 {EXTRA_FILES_DIR}/tests/tests.itb"
        name: fetch_script
        successes:
        - message: "Bytes transferred"

      - command: "source 0x48000000:test_emmc"
        name: test_emmc
        successes:
        - message: "test_emmc PASS"

      - command: "source 0x48000000:test_sdcard"
        name: test_sdcard
        successes:
        - message: "test_sdcard PASS"