[abi] define ABI x_SENDER_IDs in abi_sender_ids.h

Same BARO_BOARD_SENDER_ID for all baro_board implementations.
Since there can only be one baro_board at the same time and
this provides a good default for INS_BARO_ID.

The abi_sender_ids.h can be considered a temporary solution,
we might come up with a different scheme later...