pico-imx6ul: Add fastboot support

pico-imx6ul: Add fastboot support

fastboot tool is a convenient way to flash the eMMC, so
add support for it.

Examples of usages:

On the pico-imx6ul U-Boot prompt:

=> fastboot 0

On the Linux PC connected via USB:

1. Retrieving the U-Boot version

$ sudo fastboot getvar bootloader-version -i 0x0525
bootloader-version: U-Boot 2018.07-rc2-00130-g0881835-dirty
finished. total time: 0.000s

2. Resetting the board

$ sudo fastboot reboot -i 0x0525

(this causes the pico-imx6ul to reboot)

Signed-off-by: Fabio Estevam <[email protected]>
Signed-off-by: Fabio Berton <[email protected]>
Signed-off-by: Otavio Salvador <[email protected]>