Update doc about ways to get the config/build report

There are 2 or 3 ways to get the config/build report when the config.report file itself is not available:

• the -W command line argument
• a grep for ';;;;' on the extracted strings from the main binary
• when there is a control channel the build-report command

All have different constraints: the first requires to run the binary, the second is more hairy but requires only access to the main binary, and the last requires a running binary but can be done remotely.

Unfortunately it seems this is not explained in the ARM for all Kea commands compiled from C++ so:

• the ARM must be updated
• each way should be checked (some failures were reported and fixed so do not assume they work)