configs: fix configurations that need host-dtc

olimex_a20_olinuxino_lime_defconfig and zedboard_defconfig use U-Boot
2015.07, and they need host-dtc to build properly.

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