Board samples were listed in a very flat way before.
This commit introduces the appropriate toctrees to fix this.
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
Group all Phytec board samples in samples/boards/phytec to
keep samples/boards/ organized by vendor
Added zephyr-code-sample directive to mesh badge
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>