Improve the running-without-Makefile-patch documentation
Improve the running-without-Makefile-patch documentation

 - Include an updated list of environment variables, with sample values

 - Give a method for listing all referenced environment variables, via
   Kconfig.env_vars

 - Remove the note that says that Kconfiglib will warn if an unset
   environment variable is referenced. It's not true anymore with the
   preprocessor, which silently expands unset variables to the empty
   string.

   Not setting essential environment variables causes obvious errors at
   least.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib