CMake: Remove dependency on externally defined MBED_PATH variable

[rwalton-arm]

Description of defect

Currently we require an MBED_PATH CMake variable to be defined by the consumer in order for mbed-os to build.

It seems to be poor architecture that we require the application to define this MBED_PATH variable when mbed-os has a hard dependency on it. CMake provides several variables to automatically discover the current path, we should use them if we can. The current system leads to a non-obvious dependency on the consumer. That dependency makes building mbed-os more difficult than it should be in various cases.

If the consumer also needs to include something from mbed-os' CMake modules, e.g to run objcopy in a post build step, they should probably define that path themselves in another variable mbed-os doesn't depend on.

Target(s) affected by this defect ?

all

Toolchain(s) (name and version) displaying this defect ?

all

What version of Mbed-os are you using (tag or sha) ?

mbed-tools 7.0

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

How is this defect reproduced ?

Just look at files

[ciarmcom]

@rwalton-arm thank you for raising this issue.Please take a look at the following comments:

What target(s) are you using?
What toolchain(s) are you using?
It would help if you could also specify the versions of any tools you are using?
How can we reproduce your issue?

NOTE: If there are fields which are not applicable then please just add 'n/a' or 'None'. This indicates to us that at least all the fields have been considered.
Please update the issue header with the missing information, the issue will not be mirrored to our internal defect tracking system or investigated until this has been fully resolved.

[0xc0170]

I fixed the template, once copied internally, we can move this to CMake backlog.

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-3548