buildroot

configs/qemu_aarch64_virt_defconfig: build and use ext4 rootfs

The "virt" machine supports disk emulation, so use a ext4 rootfs instead of
initramfs for consistency with the other qemu defconfigs.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>