serial: allow external override of BSP430_I2C_SPIN_LIMIT
bmp085: fix pressure timeout
close #59: fix flush issue with I2C

I2C operations no longer return until any manually-generated STP condition
has completed.  They continue to wait for a non-busy bus before beginning
their work (the legacy check for STP at the top was a conflation of the two
needs).

Update bmp085 demo to verify that placing I2C chip into reset immediately
after use does not result in errors due to incomplete STP transmission.
close #60: fix reset issue with I2C

Clear any requested UCTX* flags in USCI control register before taking an
I2C device out of reset mode.

Extend vBSP430serialSetReset_ni() interface description so caller may
specify that peripheral must complete pending activity prior to being placed
in reset.

Delegate to vBSP430serialSetReset_ni() within vBSP430serialSetHold_ni().
sensors/sht21: add example