serprog: Add support for multiple SPI chip selects

Tested with an EN25F80, Pi Pico, and this serprog firmware:
https://codeberg.org/Riku_V/pico-serprog/commits/branch/multi-cs

Change-Id: If8052bc6f5c314dcc493bc083bb8270723efaae7
Signed-off-by: Riku Viitanen <riku.viitanen@protonmail.com>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/51
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: Nico Huber <nico.h@gmx.de>
2 files changed