buspirate_spi: Add support for hiz output with pullups=off

When working with low-voltage chips, the internal 10k pull-ups of the Bus Pirate might be too high. In such cases, it's necessary to create an external pull-up using lower-value resistors. For this, you can use the ``hiz`` parameter.  This way, the Bus Pirate will operate as an open drain with pullups=off.

So, why not just use the 'pullups=on' option for this scenario? I'm trying to prevent a very typical human error in the training sessions I conduct. For example, in the previous session, a user might have left the VPU (vextern) connected to 5V, and now they need to access a 1.8V chip. If 'pullups=on' is used, the following will happen: 5V (VPU) --> CD4060 --> 2K (internal Bus Pirate) --> MOSI target chip. By having the option to set a 'Hi-Z' mode, this can be avoided. Since the pull-ups will remain deactivated, the Bus Pirate will function in an open-drain mode with pullups=off

Change-Id: I6d9909b23782ef3fcb1428fd127dae0d6b211855
Signed-off-by: David Reguera Garcia <dreg@rootkit.es>
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M buspirate_spi.c
M doc/classic_cli_manpage.rst
2 files changed, 22 insertions(+), 2 deletions(-)