olddefconfig: Add script

Works like 'make olddefconfig', updating a configuration by filling in
default values for all new symbols.

This could also be accomplished by entering the 'menuconfig' interface
and saving the configuration, but it's more awkward and less obvious.

Piggyback two oldconfig changes:

 - Mention KCONFIG_CONFIG

 - Check if the .config file exists before parsing the Kconfig files,
   instead of after

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