Move Mbed 5 support check so that it affects the exporters

[theotherjimmy]

Description

A recent PR of mine stopped the exporters from using the build_api's
unified scan_resourcses method. Instead, I moved the supported check to
the earliest location that we have this informaiton: the end of the
config object's load_resources method.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

I suppose this code reactor allows the export command to fail much sooner in the export process?

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1637
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6512/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1267
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6512/

[mbed-ci]

Test : FAILURE

Build number : 1424
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6512/1424

[0xc0170]

@cmonr Can you review the latest test result? It's related to the patch you sent earlier, and it's on master branch?

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 1425
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6512/1425

[theotherjimmy]

@cmonr Yes, it failes before it get to the exporter. It may have even got to the compiler in the IDE before 😱

[cmonr]

@0xc0170 Am doing
@theotherjimmy xD