bif: bif-core: use static inline for all exported BIF stub functions

Mark all stub functions in the BIF consumer.h header file with
'static inline' in order to avoid compilation errors in files
that include consumer.h when CONFIG_BIF=n.

Change-Id: I3a5c7dc58f8eed93b1f30d0e021b65ab200b4271
Signed-off-by: David Collins <collinsd@codeaurora.org>