nfp: bpf: fall back to core NIC app if BPF not selected

If kernel config does not include BPF just replace the BPF
app handler with the handler for basic NIC.  The BPF app
will now be built only if BPF infrastructure is selected
in kernel config.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>