Few uboot tweaks, boot kernels over USB

Nice work on gk802 uboot! Makes this a really pleasant device to work with.

Just a couple of minor tweaks here, and addition of the ability to load kernel/initrd (or arbitrary anything) via tools/imxboot. Useful when building new kernels.

(Am being lazy and rolling them up as one pull request, feel free to cherry-pick or let me know if you'd rather I split them.)