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