DISCO_F769NI: set clock_source to USE_PLL_HSE_EXTC

[bcostm]

Description

As noticed in Issue #6549 the clock_source configuration was incorrectly set for this board. The MCU is clocked with an external clock on OSCIN pin and not using an XTAL on OSCIN/OSCOUT pins.

Tested OK on all compilers.

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1684
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6567/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1316
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6567/

[mbed-ci]

Test : FAILURE

Build number : 1478
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6567/1478

[cmonr]

Running test again. Starting to see patterns with 429ZI and lp_timer tests intermittently failing.

/morph test

[bcostm]

Running test again. Starting to see patterns with 429ZI and lp_timer tests intermittently failing.

We also noticed that since 5.8.1 (and also on master) many lptimer tests are fail now on many boards. So I don't think the failure you see here is related to this PR.

[mbed-ci]

Test : SUCCESS

Build number : 1510
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6567/1510

[cmonr]

We also noticed that since 5.8.1 (and also on master) many lptimer tests are fail now on many boards. So I don't think the failure you see here is related to this PR.

If you can think of any other issues that you think you've seen, feel free to open an issue and mention/tag/assign it to myself and/or @studavekar.