Add user interface to activate the Bus Pirate pull-up resistors.

When working with some flash chips using the Bus Pirate programmer, the
use of the Bus Pirate's on-board pull-up resistors is sometimes
necessary.

On v3 hardware the use of said pull-up resistors requires the user to apply a
voltage to the VPU pin of the Bus Pirate, and then command it to use them.
For v4 hardware which supports also fixed internal 3.3V and 5V sources no
documentation could be found.

Here is a link to information pertaining to what this patch does:
http://dangerousprototypes.com/docs/SPI_(binary)#0100wxyz_-_Configure_peripherals_w.3Dpower.2C_x.3Dpull-ups.2C_y.3DAUX.2C_z.3DCS

Bonus: small cleanup of superfluous stack variables.

Signed-off-by: Brian Salcedo <bsalcedo@gmx.us>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Tested-by: Brian Salcedo <bsalcedo@gmx.us>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1641 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1