--mbed-os-path Doesn't work

**Describe the bug**
I have a copy of the mbed os, but using the `--mbed-os-path` doesn't change the directory that cmake attempts to use; it always assumes that the os is right in the current directory.

**To Reproduce**
Steps to reproduce the behavior:
1. Run `mbedtools new PATH` to download the mbed os.
2. Run `mbedtools new -c OS_PATH` to create a project without copying the entire operating system.
3. Run `mbedtools compile [...] --mbed-os-path OS_PATH`
4. See error

**Expected behavior**
The `--mbed-os-path` setting should modify the `MBED_PATH` so that cmake is pointed to the correct os directory.

Start at the mbedtools compile command and trace how --mbed-os-path is passed into the CMake invocation. Reproduce the documented mbedtools new and compile steps with a separate OS_PATH, then verify that CMake receives an MBED_PATH pointing to that directory instead of the current directory.

Written by the indexing model from the issue text.