Clean up main a bit by using fnet_init_static, and putting fnet_cpu_irq_enable(0) in init_fnet()

fnet_init_static uses a static buffer internally allocated instead of being passed one. Makes a lot of sense to use it as it just does what we are doing already, but more clearly.
fnet_cpu_irq_enable naturally goes with fnet_init as well, no point having it seperate.