move lvgl and lv_drivers

Ideally both would be in subprojects, but due to how they expect to be
built, and how my hacks work, lv_drivers is better suited at the top
level here.

This is only temporary as we hope to drop lv_drivers when upgrading to
newer lvgl.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>