The board name for the Thingy:52, so far known as nrf52_pca20020, is
renamed to thingy52_nrf52832. Its documentation and all references to
its name in the tree are updated accordingly. Overlay and configuration
files specific to this board are also renamed, to match the new board
name.
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>