acinclude: allow configuration for kernel version 3.18

Kernel module does not compile with kernel version 3.18. For now the
kenrel modules will be developed on net-next. This change will allow
datapath/bpf to be configured on 3.18 kernel.
bpf: add basic datapath/bpf directory to host kernel bpf programs

Should generate both bpf binary (*.bpf) and bpf assembly (*.bps)
All source code in *.c will be compiled as bpf programs