Bluetooth: Assert alignof bt_addr types

I got some feedback about confusion as to why the bt_addr types are not
packed. This commit adds a note about it.

This commit also adds a check for the alignof of the structs to verify
the statement 'their members are bytes or byte arrays' to thoroughly
prove that the struct is effectively as-if packed.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>