soc: esp32: use SoC name as IDF target instead of board name

So far board name was used as IDF target. This worked, as the only board
in tree that is based on 'esp32' SoC is actually 'esp32'.

Use CONFIG_SOC instead of CONFIG_BOARD, so that new boards based on
'esp32' SoC can be successfully introduced both downstream or upstream.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>