d/dummy: Add dummy HMD driver

CMakeLists.txt

meson.build

meson_options.txt

src/CMakeLists.txt

src/meson.build

src/targets_enabled_drivers.h.cmake_in

src/targets_enabled_drivers.h.meson_in

src/xrt/drivers/CMakeLists.txt

src/xrt/drivers/dummy/dummy_hmd.c

src/xrt/drivers/dummy/dummy_interface.h

src/xrt/drivers/dummy/dummy_prober.c

src/xrt/drivers/meson.build

src/xrt/targets/CMakeLists.txt

src/xrt/targets/common/target_lists.c

src/xrt/targets/meson.build