zedboard: add Avnet Zedboard support.

[Thomas: update to recent Buildroot which had renamed several
Config.in options, lock down kernel headers version]

Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>