bpf: remove use of weak functions

bpf: remove use of weak functions

Weak functions don't work well with static libraries and require the use of
"whole-archive" flag to ensure that the correct function is used when
linking.  Since the weak function is only used as a placeholder within this
library alone, we can replace it with a non-weak version protected using
preprocessor ifdefs.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Tested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>