buildroot

configs/qemu: update sparc ss10

Switch to kernel 4.0.
Switch kernel config to ext4, simplify to reduce bloat.
Adjust invocation to keep the framebuffer window for graphics testing.

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