examples/telnet_server: add support for LWIP

[benpicco]

Contribution description

Just enables the telnet example to be build with LWIP.

Testing procedure

Just build the examples/telnet_server with LWIP=1

Output is a bit broken compared to GNRC:

% telnet fe80:0:0:0:7837:fcff:fe7d:1aaf%tapbr0
Trying fe80::7837:fcff:fe7d:1aaf%tapbr0...
Connected to fe80:0:0:0:7837:fcff:fe7d:1aaf%tapbr0.
Escape character is '^]'.
RIOT telnet example application╔═══════════════════════════════════════════════════╗║telnet is entirely unencrypted and unauthenticated.║║Do not use this on public networks.                ║╚═══════════════════════════════════════════════════╝ps
  
p
s


	pid | name                 | state    Q | pri | stack  ( used) ( free) | base addr  | current     

	  - | isr_stack            | -        - |   - |   8192 (   -1) ( 8193) |  0x8084880 |  0x8084880

	  1 | idle                 | pending  Q |  15 |   8192 ( 1084) ( 7108) |  0x807d180 |  0x807efe0 

	  2 | main                 | running  Q |   7 |  12288 ( 3004) ( 9284) |  0x807f180 |  0x8081fe0 
	  3 | lwip_netdev_mux      | bl rx    _ |   3 |   8192 ( 1264) ( 6928) |  0x808d0a0 |  0x808ef00 
	  4 | tcpip_thread         | bl mbox  _ |   1 |   8192 ( 1756) ( 6436) |  0x8088d38 |  0x808ab98 
	  5 | telnet               | sleeping _ |   6 |   8192 ( 2580) ( 5612) |  0x808f820 |  0x8091680 
	    | SUM                  |            |     |  53248 ( 9688) (43560)
> 
> 
  >> 

which hints at some inconsistency on how the two TCP stacks are integrated.

Issues/PRs references

[maribu]

Output is a bit broken compared to GNRC:

Huh? That sounds like data gets corrupted in transit. Maybe there is an off-by-one issue that the last byte of a chunk is not send?

[maribu]

ACK. Change is unscary and I trust your testing