configs: add lego_ev3_defconfig

configs: add lego_ev3_defconfig

Add initial support for the Lego Mindstorms EV3 programmable brick.

The Lego Mindstorms EV3 brick comprises a Texas Instruments AM1808 SoC, with
an ARM 926EJ-S main processor running at 300 MHz.
https://en.wikipedia.org/wiki/Lego_Mindstorms_EV3

This configuration uses the Linux kernel of the ev3dev project.
https://github.com/mindboards/ev3sources

More info is available in the board/lego/ev3/readme.txt file, shamelessly
documented in the same way as the SoCkit folks did.

[Peter: lock kernel headers to match]
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>