west: promote config_get*() functions from sign.py to the base class

These two functions have stood the test of the time and they have
absolutely nothing specific to sign.py

This has the benefit of transitioning away from west's global and
deprecated logging interface
(https://github.com/zephyrproject-rtos/west/issues/149

) and this
deprecation is what prompted this commit: see #79240.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>