buspirate_spi: Add support for variable serial speeds

This patch sets the default baud rate for communication between
the host device and the Bus Pirate for hardware versions 3.0
and greater to 2M baud.

It also introduces the ability to manually set the baud rate via
the added 'serialspeed' programmer parameter.

This is done in two parts. Firstly, the requested serial speed is looked up
in a table to determine the appropriate clock divisor and the divisor is sent
to the bus pirate. Then, the system's baud rate for the selected serial port
is set using serial.c's 'serialport_config'. This function's prototype had to
be added to programmer.h.

In testing, using the 2M baud rate was able to significantly decrease
flash times (down from 20+ minutes to less than 2 minutes for an 8MB flash).

Change-Id: I3706f17a94fdf056063f2ad4a5f0a219665cdcbf
Signed-off-by: Shawn Anastasio <shawnanastasio@yahoo.com>
---
M buspirate_spi.c
M flashrom.8.tmpl
M programmer.h
3 files changed, 136 insertions(+), 9 deletions(-)