add support to silence deprecation warning for easyconfigs and toolchains

[Flamefire]

We have the silence_deprecation_warnings option but that didn't support the warnings caused by deprecated ECs and toolchains.

Add this and enhance the error message.

While at it also make the build_option return an empty list instead of None so we can grep the code for any usage like if 'foo' in build_option('silence_deprecation_warnings'):

[Flamefire]

This now adds a new change:

Only print "default: " for str-list options if it is not empty.
This changes (type comma-separated list; default: ) to (type comma-separated list) similar to a default of None.

That changes the output of --help in the following places (done using diff over the output before and after):

                    Parse (additional) configfiles (type comma-separated list)
                    Enable dependency resolution, using easyconfigs in specified paths (type pathsep-separated list)
                    Additional paths to consider by robot for easyconfigs (--robot paths get priority) (type pathsep-separated list)
                    Additional locations to consider in --search (next to --robot and --robot-paths paths) (type pathsep-separated list)
                    Location(s) of extra or customized easyblocks (type comma-separated list)
                    Location(s) of extra or customized module naming schemes (type comma-separated list)
                    Location(s) of extra or customized toolchains or toolchain components (type comma-separated list)
--from-pr=PR#       Obtain easyconfigs from specified PR (type comma-separated list)
                    Include easyblocks from specified PR (type comma-separated list)
                    Accept EULA for specified software [DEPRECATED, use --accept-eula-for instead!] (type comma-separated list)
                    Accept EULA for specified software (type comma-separated list)
                    Silence specified deprecation warnings out of (python2, Lmod6, easyconfig, toolchain) (type comma-separated list)

This can be discussed (if needed) in a separate PR I just opened with only that commit: #4220

[jfgrimm]

lgtm