boot/uboot: add option to pass custom variables to U-Boot build

boot/uboot: add option to pass custom variables to U-Boot build

U-Boot supports a number of environment variables to pass specific
information. The following patches were submitted in the past to one
some specific Config.in option to pass some of these variables:

 - http://patchwork.ozlabs.org/patch/881197/ proposed an option to
   pass a custom EXT_DTB= variable

 - http://patchwork.ozlabs.org/patch/1018245/ proposed an option to
   pass a custom DEVICE_TREE= variable

Instead of adding one Config.in option for each of those variables,
let's provide a generic mechanism to pass arbitrary variables during
U-Boot build step.

Cc: Konstantin Porotchkin <kostap@marvell.com>
Cc: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>