Add an option to take a Kconfig file fragment as a constraints file

[paulgazz]

Currently, --constraints-file uses a custom format. Add a new option that can take constrained configuration options in the Kconfig format to make it easier to combine Kconfig configuration fragments.

[paulgazz]

One possibility is to add a new --include support for .config files, so that both --include (required options) and --include-mutex (required options that may have contradictions) options can be used to create config files from config fragments.