Add configurable Max Message Size for TinyOS

[gitbaum]

Add configurable max message data size to the generic makelfiles for TinyOS. Just add -DTOSH_DATA_LENGTH=x as a commandline option to ncc.

Thanks to Peter Rothenpieler for the link:
http://www.tinyos.net/tinyos-2.x/doc/html/tep111.html

Interesting part:
"""
3.2 Data

The data field of message_t stores the single-hop packet payload. It
is TOSH_DATA_LENGTH bytes long. The default size is 28 bytes. A
TinyOS application can redefine TOSH_DATA_LENGTH at compile time with
a command-line option to ncc: -DTOSH_DATA_LENGTH=x. Because this
value can be reconfigured, it is possible that two different versions
of an application can have different MTU sizes. If a packet layer
receives a packet whose payload size is longer than TOSH_DATA_LENGTH,
it MUST discard the packet. As headers are right justified to the
beginning of the data payload, the data payloads of all link layers
on a platform start at the same fixed offset from the beginning of
the message buffer.
"""