cnss: Add the support to load a seprate board data file for FTM mode

Currently FTM mode uses the same board data file as normal mode, but
for some cases it may require a different board data file, hence add
the changes to support it.

Change-Id: I0c74ddc9ee3b0abc949db388f0fa48d763dd5fb2
CRs-Fixed: 637918
Signed-off-by: Yue Ma <yuem@codeaurora.org>