mbed-tools configure fails due to multiple mbed-os paths being searched

[OllyMcBrideOB]

Issue
The same error as #234 (see error message below), possibly caused by using a shared instance of mbed-os.

Error
ValueError: Setting <random configuration setting> already defined. You cannot duplicate config settings!
(The setting name changes every time the command is executed)

Details
I believe this issue is caused when assemble_config() finds the mbed-os/ path within program.root, and then the mbed-os/ path is also passed in via program.mbed_os.root.

mbed-tools/src/mbed_tools/build/config.py

Lines 36 to 38 in 519d3ec

	config = assemble_config(
	target_build_attributes, [program.root, program.mbed_os.root], program.files.app_config_file
	)

In the above snipped, if I remove program.mbed_os.root everything works correctly.

To Reproduce

1. Navigate to mbed-os-example-blinky
2. Use shared instance of mbed-os
3. Enter mbed-tools configure -m NRF52_DK -t GCC_ARM

Expected behavior
The mbed-tools configure -m NRF52_DK -t GCC_ARM command should configure the mbed-os-example-blinky project for CMake.

Desktop (please complete the following information):

• OS: Windows
• Version: 21H1

Mbed (please complete the following information):

• Device: NRF52_DK
• Mbed OS Version: [e.g. 6.8.0]
• Mbed CLI 2 Version: 7.50.0