Made library for ring buffer mapped to a file.

Storing head/tail info at the end of the file too.
Started writing code for Bluetooth I/O.

Now uses `pthreads`.
Tried to make `ring_buf` a little thread safe.
Added update command under installing packages

instruction was missing opkg update command
Fixed typo so that tutorial matches path on device

sc_mod_test path should be ~/isoblue-software/tools/ not ~/isoblue-software/tool
Fixed filter setting bug in ISOBUS module.
Added filtering test/example to `sc_mod_test.c`.
Filtering works now, threading was changed.

Also changes thread stuff:
* Shouldn't lose commands anymore
* Should prevent only 1 of in/out thread dying