6LoWPAN: update documentation

See header include/net/mac802154.h and several drivers in drivers/ieee802154/.

6LoWPAN Linux implementation

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The IEEE 802.15.4 standard specifies an MTU of 128 bytes, yielding about 80

octets of actual MAC payload once security is turned on, on a wireless link

with a link throughput of 250 kbps or less.  The 6LoWPAN adaptation format

[RFC4944] was specified to carry IPv6 datagrams over such constrained links,

taking into account limited bandwidth, memory, or energy resources that are

expected in applications such as wireless Sensor Networks.  [RFC4944] defines

a Mesh Addressing header to support sub-IP forwarding, a Fragmentation header

to support the IPv6 minimum MTU requirement [RFC2460], and stateless header

compression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the

relatively large IPv6 and UDP headers down to (in the best case) several bytes.

In Semptember 2011 the standard update was published - [RFC6282].

It deprecates HC1 and HC2 compression and defines IPHC encoding format which is

used in this Linux implementation.

All the code related to 6lowpan you may find in files: net/ieee802154/6lowpan.*

To setup 6lowpan interface you need (busybox release > 1.17.0):

1. Add IEEE802.15.4 interface and initialize PANid;

2. Add 6lowpan interface by command like:

   # ip link add link wpan0 name lowpan0 type lowpan

3. Set MAC (if needs):

   # ip link set lowpan0 address de:ad:be:ef:ca:fe:ba:be

4. Bring up 'lowpan0' interface