doc: scobc_a1: Explain OpenOCD interface selection

Explain how to use CMSIS-DAP, FT2232D, or FT232R interfaces by setting the
OPENOCD_INTERFACE environment variable when flashing the SC-OBC Module A1.

This reflects the recent unification of OpenOCD configuration scripts and
the default selection of CMSIS-DAP as the standard interface.

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>