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 Google board samples in samples/boards/google to
keep samples/boards/ organized by vendor
Added zephyr-code-sample directive to twinkie sample
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>