Reland "Remove the 'gyp_config' concept from MB."

tools/mb/docs/user_guide.md

 # The MB (Meta-Build wrapper) user guide
 
 [TOC]
 
 ## Introduction
 
 `mb` is a simple python wrapper around the GYP and GN meta-build tools to
 be used as part of the GYP->GN migration.
 
 It is intended to be used by bots to make it easier to manage the configuration
@@ skipping 154 matching lines @@
 configuration commonly used by develpers but that we may not have a bot
 for.
 
 The value of each key is a list of "mixins" that will define what that
 build_config does. Each item in the list must be an entry in the dictionary
 value of the `mixins` key.
 
 Each mixin value is itself a dictionary that contains one or more of the
 following keys:
 
-  * `gyp_configs`: a list of the configurations to build, e.g.,
-    ['Release', 'Release_x64'].
   * `gyp_crosscompile`: a boolean; if true, GYP_CROSSCOMPILE=1 is set in
     the environment and passed to GYP.
   * `gyp_defines`: a string containing a list of GYP_DEFINES.
   * `gn_args`: a string containing a list of values passed to gn --args.
   * `mixins`: a list of other mixins that should be included.
   * `type`: a string with either the value `gyp` or `gn`;
     setting this indicates which meta-build tool to use.
 
 When `mb gen` or `mb analyze` executes, it takes a config name, looks it
 up in the 'configs' dict, and then does a left-to-right expansion of the
-mixins; gyp_defines and gn_args values are concatenated, and type and
-gyp_configs values override each other.
+mixins; gyp_defines and gn_args values are concatenated, and the type values
+override each other.
 
 For example, if you had:
 
 ```
 {
   'configs`: {
     'linux_release_trybot': ['gyp_release', 'trybot'],
     'gn_shared_debug': None,
   }
   'mixins': {
     'bot': {
       'gyp_defines': 'use_goma=1 dcheck_always_on=0',
       'gn_args': 'use_goma=true dcheck_always_on=false',
     },
     'debug': {
       'gn_args': 'is_debug=true',
     },
     'gn': {'type': 'gn'},
     'gyp_release': {
-      'gyp_config': 'Release'
       'mixins': ['release'],
       'type': 'gyp',
     },
     'release': {
       'gn_args': 'is_debug=false',
     }
     'shared': {
       'gn_args': 'is_component_build=true',
       'gyp_defines': 'component=shared_library',
     },
@@ skipping 19 matching lines @@
 run than you expect; running `'mb -v'` will print what it's doing and
 run the commands; `'mb -n'` will print what it will do but *not* run
 the commands.
 
 If you hit weirder things than that, add some print statements to the
 python script, send a question to gn-dev@chromium.org, or
 [file a bug](https://crbug.com/new) with the label
 'mb' and cc: dpranke@chromium.org.