mbed config not properly documented

[TacoGrandeTX]

@theotherjimmy As we discussed, mbed config should:

• accept any variable even if useless
• produce proper output for --help - it doesn't correctly list the variables that actually have an affect

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-943

[theotherjimmy]

We discussed that mbed config should only accept the variables that it understands.

[naveenkaje]

Submitted #865