buildroot

configs/warp7: Bump to mainline kernel 4.9

Use mainline 4.9 instead of a custom kernel based on NXP 4.1.

As mx7 boots in non-secure mode in mainline kernel, change the
U-Boot target to "warp7".

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>