buildroot

board: raspberrypi: use regular kernel image

Since release 4.4 a kernel without a trailer is assumed to be device tree
capable. Since our Raspberry Pi defconfigs all use the newer firmware we can
just use the regular kernel image.

https://www.raspberrypi.org/documentation/configuration/device-tree.md

Tested on Raspberry Pi 3.

Cc: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>