buildroot

configs/qemu: update arm vexpress

Switch to kernel 4.0.
Switch to DTB (mandatory).
And adjust invocation to use dtb and add a little more RAM for testing.

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