Test build: Reduce include command length

[theotherjimmy]

Description

Resolves #6577

Tests are built in two passes:

• Mbed OS as a lib (just .o files)
• Each test is built and linked with the Mbed OS .o

The second pass involves scanning the build directory creating in the first
pass. This is where the bug lies: the second pass takes an absolute path to
the artifacts create by the first. This adds the directory containing mbed-os
to the each include path, and these extra characters exceed the maximum
command line length on windows.

This patch uses a relate path, eliminating the problem

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2096
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6968/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 1906
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6968/1906

[mbed-ci]

Exporter Build : FAILURE

Build number : 1737
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6968/

[cmonr]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1740
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6968/