Commit related to null pointer exception detection causing UART issues

[torsteingrindvik]

Describe the bug

I have been seeing issues related to UART lately.
The current master (8bee027ec4) has the following problem:

• UART works right after flashing
• If the board is reset (nrfjprog -r), no output is shown

I have only found this to be true in tests, not in samples.
I found that the bug happens when ZTEST is used.

I have bisected the current master (bad) vs. the v2.5.0/fe7c2ef tag/commit (good), and found the likely causal commit to be: 1b22f6b8c8.
This commit ("enable null-pointer exception detection in the tests") introduces CONFIG_CORTEX_M_DEBUG_NULL_POINTER_EXCEPTION.
If the below steps to reproduce are repeated but with CONFIG_CORTEX_M_DEBUG_NULL_POINTER_EXCEPTION_DETECTION_NONE=y (since we are not allowed to set the former manually), the problem goes away (UART appears even when restarting).

To Reproduce

I found a simple test which includes printing stuff to UART.
(This test uses debug.conf instead of the implicit prj.conf, so it must be explicitly passed)

1. cd tests/misc/test_build
2. west build -b nrf52840dk_nrf52840 --pristine -- -DOVERLAY_CONFIG=debug.conf
3. Notice the continous UART output
4. nrfjprog -r
5. Notice the lack of any UART output

Expected behavior
UART output also when board is reset.

Impact
Is likely a cause for some of the CI issues in nRF Connect SDK downstream sdk-nrf seen the last several weeks (for tests relying on UART).

Environment (please complete the following information):

• OS: Linux
• Toolchain: gnuarmemb, 9 q4

[jfischer-no]

related to #33381

[galak]

I think we should disable this be default. Users keep running into this and I think their expectation of being able to debug should take precedence.

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