sfxge: support variable-length response to MCDI GET_BOARD_CFG

Allocate the minimum or maximum response length for GET_BOARD_CFG as
appropriate. When looking up firmware subtypes by partition ID,
check the ID against the actual response length.

Merge of the patch made by Ben Hutchings in 2011.

Sponsored by: Solarflare Communications, Inc.
Approved by: gnn (mentor)