Add USB loaders for sunxi, tegra, samsung

[sjg20]

This adds USB loaders for three different SoCs. The Tegra loader is fairly straightfoward and similar to iMX, so it can use the same protocol.

For Allwinner (sunxi) and Samsung, the loading happens in 2-3 phases, so a new 'phase' parameter is added to the protocol. This allows USB loading to work correctly on these boards, sending an initial BL1 image (in the case of Samsung), then U-Boot SPL and U-Boot proper.

This has been tested on:

• snow (exynos5250)

• Linksprite_pcDuino3 (sunxi)

• nyan-big (Tegra124-based Chromebook)

• [X] Documentation for the feature

• [ ] Tests for the feature

• [X] The arguments and description in doc/configuration.rst have been updated

• [X] PR has been tested

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.15152% with 41 lines in your changes missing coverage. Please review.

Project coverage is 55.9%. Comparing base (4398da7) to head (c56f683).

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/usbloader.py	83.9%	17 Missing :warning:
labgrid/resource/udev.py	50.0%	12 Missing :warning:
labgrid/remote/client.py	0.0%	9 Missing :warning:
labgrid/resource/suggest.py	0.0%	3 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1482     +/-   ##
========================================
+ Coverage    55.6%   55.9%   +0.2%     
========================================
  Files         172     172             
  Lines       13471   13634    +163     
========================================
+ Hits         7500    7628    +128     
- Misses       5971    6006     +35     

Flag	Coverage Δ
3.10	55.9% <75.1%> (+0.2%)	:arrow_up:
3.11	55.9% <75.1%> (+0.2%)	:arrow_up:
3.12	55.9% <75.1%> (+0.2%)	:arrow_up:
3.13	55.9% <75.1%> (+0.2%)	:arrow_up:
3.9	55.9% <75.1%> (+0.2%)	:arrow_up:

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[sjg20]

Hi, could this patch be merged please?