Allowing root directory configuration

[bridadan]

As of 1.0.0, you can no longer set the root directory of program that doesn't already have a .mbed file, which kind of defeats the purpose of the option :)

This patch lets all local changes to the ROOT config option through the config check.

It also changes the error message printed for any other variable that is trying to be modified to include this way forward.

The error before was the following:

C:\Users\bridan01\Documents\dev\m_mbed\mbed>mbed target K64F
[mbed] ERROR: Could not find mbed program in current path "C:\Users\bridan01\Documents\dev\m_mbed\mbed".
[mbed] ERROR: Change the current directory to a valid mbed program or use the '--global' option to set global configuration.

Now it is the following:

C:\Users\bridan01\Documents\dev\m_mbed\mbed>mbed target K64F
[mbed] ERROR: Could not find mbed program in current path "C:\Users\bridan01\Documents\dev\m_mbed\mbed".
[mbed] ERROR: Change the current directory to a valid mbed program, set the current directory as an mbed program with 'mbed config root .', or use the '--global' option to set global configuration.

Please review @screamerbg

[teetak01]

Fixes our issues. We have been using "mbed config root ." as part of build-scripts.

[bridadan]

Had an offline discussion with @bremoran and it looks like this would fix an issue he's having issue as well.

[bridadan]

@screamerbg bump!