configs: atmel: at91sam9260eknf: update defconfig

Bump to a recent version of AT91bootstrap and use mainline version of
U-Boot and Linux. This is needed to fix the build with gcc 5.x.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
[Thomas: add missing BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_8=y.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>