fastboot: Add support for flashing zImage

This patch adds support for flashing zImage to the Android boot
partition on eMMC.

Usage:

    $ fastboot flash zImage <path_to_zImage>

It's based on [1].

[1] http://omapzoom.org/?p=repo/u-boot.git;a=commit;h=3393b908c1e848bba3706612cbe50aa8970720b3



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>