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.