Usage: set QEMU_PSS_ENABLE=y and adjust QEMU_PSS_HOST_DIR. It also
requires QEMU_VIRTFS_ENABLE to be set to "y".

Also added a buildroot post-script which appends lines to /etc/fstab,
so shared directories can be automatically mounted if
QEMU_VIRTFS_AUTOMOUNT and QEMU_PSS_AUTOMOUNT are set to "y".

Signed-off-by: Roland Nagy <rnagy@xmimx.tk>
Reviewed-by: Jerome Forissier <jerome@forissier.org>
Tested-by: Jerome Forissier <jerome@forissier.org>