buildroot

configs/qemu: update aarch64 to linux 4.6.3

The console problem goes away with qemu 2.6.0 so it seems it's now a
requirement, update readme.txt accordingly.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>