Custom targets were the origin of a number of issues with the mbed
tools, so it was decided that they need to be removed. This PR does just
that (and moves the "custom_targets" part of the config system tests
into a separate, per-test "targets.json" file to preserve the test
functionality).
The configuration system uses `mbed_config.h` to export configuration
data, but when this change was made, the documentation was not properly
updated. This commit updates the documentation with the correct
behaviour of the configuration system.