boards: esp32: Add note about fetching an older version of ESP-IDF

ESP-IDF is in constant development and it's likely that files Zephyr
depends on will be moved, removed, or renamed.  Make a note that an
older version of ESP-IDF should be used instead.

Closes #1538.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>
1 file changed