config toolchain - need to list options

[BlackstoneEngineering]

Problem

nowhere in the mbed-cli help menus does it list what the acceptable options are for mbed config toolchain

Solution

1. add a complete list to the help flavor text for toolchain
2. add tab completion and allow user to cycle thorugh list with tab completion

[BlackstoneEngineering]

I think @theotherjimmy has some pending PR's that would make tab completion rediculously simple to add.

[theotherjimmy]

Well, no. It's nowhere near ridiculously simple.

[screamerbg]

Tab completion is a duplicate of #241. Please read @mjs-arm and @bremoran's comments.

[BlackstoneEngineering]

Oi, dont close this, this is a super valid bug! This bug isnt about addint tab completion, its about adding a way to easily specify the toolchain. The other ticket is about git, which is wrapping another tool, this is about our own provided options.

[screamerbg]

$ mbed toolchain -h
usage: mbed toolchain [-h] [-G] [-S] [-v] [-vv] [name]

Set or get default toolchain
This is an alias to 'mbed config [--global] toolchain [name]'

positional arguments:
  name                 Default toolchain name. Example: ARM, uARM, GCC_ARM, IAR

[theotherjimmy]

Closing, as "a way to easily specify toolchain" is already present, as shown above.