NUCLEO_L432KC: Fixed async serial

[MikeDK]

Description

In targets.json there is no SERIAL_ASYNC defined for the NUCLEO_L432KC developer board.
When adding this option, a bug in targets/TARGET_STM/TARGET_STM32L4 was encountered which was leading to compile errors.

[MikeDK]

PS: sorry about the duplicate pull request ... i was not able to remove the unnecessary merge commit ... i did a reset to the mbed-os master, which removed my commits and thus automatically closed the previous pull request ;)

[0xc0170]

@MikeDK Thanks ! Good to see that coming from a branch, you can now use master to follow upstream/master 💯

[0xc0170]

@ARMmbed/team-st-mcd Please review

[0xc0170]

@MikeDK any tests done with this fix?

[0xc0170]

/morph test

[MikeDK]

@0xc0170 Yes I did some testing with the logger class I wrote for our project. The class supports blocking and non-blocking output (the latter is useful for logging something from within interrupt context) and both is working like a charm with the fix.

[theotherjimmy]

/morph build.

[theotherjimmy]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 561
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5535/

Triggering tests

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

[mbed-ci]

Build : SUCCESS

Build number : 561
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5535/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 174
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5535/

[mbed-ci]

Test : SUCCESS

Build number : 375
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5535/375