buildroot

configs/qemu: update ppc virtex ml507

Switch to kernel 4.0.
Use a custom kernel config to avoid the caveat of a huge debug-enabled
one.
And also reduce defconfig options to avoid bloat.

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