Remove ancient and outdated references to config.mk

Author: jyn514

README.md

@@ -228,9 +228,7 @@ configure script and makefile (the latter of which just invokes `x.py`).
 make && sudo make install
 ```
 
-When using the configure script, the generated `config.mk` file may override the
-`config.toml` file. To go back to the `config.toml` file, delete the generated
-`config.mk` file.
+`configure` generates a `config.toml` which can also be used with normal `x.py` invocations.
 
 ## Building Documentation
 

src/bootstrap/README.md

@@ -80,18 +80,12 @@ The script accepts commands, flags, and arguments to determine what to do:
 
 ## Configuring rustbuild
 
-There are currently two methods for configuring the rustbuild build system.
-
-First, rustbuild offers a TOML-based configuration system with a `config.toml`
+rustbuild offers a TOML-based configuration system with a `config.toml`
 file. An example of this configuration can be found at `config.toml.example`,
 and the configuration file can also be passed as `--config path/to/config.toml`
 if the build system is being invoked manually (via the python script).
 
-Next, the `./configure` options serialized in `config.mk` will be
-parsed and read. That is, if any `./configure` options are passed, they'll be
-handled naturally. `./configure` should almost never be used for local
-installations, and is primarily useful for CI. Prefer to customize behavior
-using `config.toml`.
+You can generate a config.toml using `./configure` options if you want to automate creating the file without having to edit it.
 
 Finally, rustbuild makes use of the [cc-rs crate] which has [its own
 method][env-vars] of configuring C compilers and C flags via environment

src/bootstrap/config.rs

@@ -46,10 +46,7 @@ pub enum DryRun {
 
 /// Global configuration for the entire build and/or bootstrap.
 ///
-/// This structure is derived from a combination of both `config.toml` and
-/// `config.mk`. As of the time of this writing it's unlikely that `config.toml`
-/// is used all that much, so this is primarily filled out by `config.mk` which
-/// is generated from `./configure`.
+/// This structure is parsed from `config.toml`, and some of the fields are inferred from `git` or build-time parameters.
 ///
 /// Note that this structure is not decoded directly into, but rather it is
 /// filled out from the decoded forms of the structs below. For documentation