Make clock source changeable over mbed_app.json for EFM32-Targets

[DBS06]

By adding the missing include the clock source for EFM32-Targets is changeable over mbed_app.json.

Description

Referencing issue #7346. Because nothing happens by silicon-labs regarding this issue I created this PR.

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2492
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7386/

Triggering tests

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

[stevew817]

LGTM, thanks @DBS06 for the PR.

[mbed-ci]

Test : SUCCESS

Build number : 2268
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7386/2268

[mbed-ci]

Exporter Build : FAILURE

Build number : 2134
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7386/

[0xc0170]

/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 2144
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7386/

[cmonr]

/morph export-build

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 2156
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7386/

[mbed-ci]

Exporter Build : FAILURE

Build number : 2156
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7386/

[0xc0170]

We are experiencing CI machine error, investigating and restarting once fixed

cc @OPpuolitaival @studavekar

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2169
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7386/