package: qemu: bump version to 2.8.0

package: qemu: bump version to 2.8.0

This adds a CPU definition for the Cortex A7 along with improvements described
here: http://wiki.qemu-project.org/ChangeLog/2.8

Tested on an ARM Cortex A7 target (both target and host builds). The change log
does not describe any incompatible changes that would affect buildroot targets
as far as I am aware.

Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>