add tools/imxboot: a USB HID bootloader for i.MX6

This lets you boot .imx images directly over USB. Entering USB boot mode
is board-dependent; on the GK802 it suffices to remove both SD cards.

Original by James Laird <jhl@mafipulation.org>, arbitrary file load
functionality by Angus Gratton <gus@projectgus.com>.
mx6: add support for EHCI host on OTG port
arm: add board_recovery_init_f() boot hook

This is necessary to implement recovery-switch functionality that wishes
to rewrite environment variables.
No existing hook is suitable, because this must execute after the
environment is relocated (to write environment vars) but before the
console (if it is to reset video-related vars).
cfb_console: fix cache-related draw bugs & compile warning
Add support for GK802 HDMI stick computers, designed by Richtechie.