A single command to build an example from scratch

From @thegecko:

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
Please can I ask that the mbed-tools cli has an extra flag/switch added to automatically hydrate a project before build? e.g. `mbed-tools build -m K64F -t ARM --checkout`
That way a single command can be used to build an example from scratch.

**Describe alternatives you've considered**
I could run two commands at once. `mbed-tools deploy && mbed-tools build -m K64F -t ARM`

**Additional context**
None

Start by inspecting the existing mbed-tools build and deploy command entry points and their command tests. Trace how deploy hydrates a project before build, then define the expected behavior for `mbed-tools build -m K64F -t ARM --checkout`. Done means an example can be hydrated and built successfully with that single command, while the existing two-command workflow remains valid.

Written by the indexing model from the issue text.