pkg: add nanocoap

[kaspar030]

Here's the first PR regarding nanocoap.
While nanocoap contains client functionality, that depends on sock, so I've decided to add that at a later stage.

The PR includes an example application that has almost exactly the same functionality as microcoap_example (it's also based on conn for now).

Here's a code/memory comparison:

[kaspar@booze nanocoap_server (add_pkg_nanocoap)]$ BOARD=samr21-xpro make info-objsize | grep -E '(coap|main)'
    944       0       0     944     3b0 nanocoap.o (ex nanocoap.a)
    144       0     512     656     290 nanocoap_conn.o (ex nanocoap_server.a)
    178       0      64     242      f2 main.o (ex nanocoap_server.a)
    150       0       0     150      96 coap_handler.o (ex nanocoap_server.a)
[kaspar@booze nanocoap_server (add_pkg_nanocoap)]$

[kaspar@booze microcoap_server (add_pkg_nanocoap)]$ BOARD=samr21-xpro make info-objsize | grep -E '(coap|main)'
    164       8    1536    1708     6ac microcoap_conn.o (ex microcoap_server.a)
   1244       0       0    1244     4dc coap.o (ex microcoap.a)
    468       0     500     968     3c8 coap.o (ex microcoap_server.a)
    179       0      64     243      f3 main.o (ex microcoap_server.a)
[kaspar@booze microcoap_server (add_pkg_nanocoap)]

Thats 1416b code / 576b RAM for nanocoap and 2055b code / 2108b RAM for microcoap.
The comparison is favoring nanocoap, as microcoap currently has a little more functionality (it adds content types to /.well-known/core), and its buffers could probably be made smaller. Then again, it is not easy to determine how big they should be for microcoap, which was one of the main reasons why I started the rewrite.

Also, nanocoap only needs 57 lines of CoAP specific SLOC for this simple example, while microcoap needs ~130.

[kaspar030]

(launching CI to check for build errors)

[miri64]

Needs port to sock ;-)

[kaspar030]

done. :)

[miri64]

Consider making this only a host #6007

[kaspar030]

Will do in a follow-up.

[kaspar030]

• squashed, as it now should build and I won't be able to change things until I'm back from holidays.

[miri64]

Tested with Copper on native (and the provided example). ACK!

[miri64]

and go.