sDDF LWIP cleanup

[Ivan-Velickovic]

As pointed out by @jearc, the LWIP code from the echo server is fairly tangled with the rest of the code-base which is becoming not ideal as we use LWIP outside the echo server context and inside the Kitty example.

The following is a list of things that should be cleaned up:

• sddf/util/timer.h is LWIP specific
• sddf/network/componenets/lwip_timer.c is LWIP specific too
• Build an LWIP static library - have a Makefile that takes in the client-specific LWIP options to make it easier to build a network client, right now the process to compile LWIP is tedious.
• Move lwip.c to echo_server.c and have sddf/network/components/lwip.c be generic LWIP code that does not depend on the client - e.g there should be no notified entry point for lwip.c.

[Ivan-Velickovic]

Made au-ts/sddf#57 instead.