buildroot

configs/warp7: Use mainline U-Boot

warp7 is well supported in mainline U-Boot 2016.09, so switch
to this version.

As the current supported kernel is based on a 4.1 kernel from NXP,
use the "warp7_secure" U-Boot target as NXP kernel needs to boot
in secure mode.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>