[TF-M] halting or rebooting on faults

Hi,

it seems we could try to be more consistent with fault handling.

The current default behaviour is:

Error code returned from an SPE function? Reboot. MemFault/HardFault/SecureFault in the SPE? Halt.

Null-pointer dereference from the NSPE? (results in a secure fault for cortex-m) Halt.

Should we perhaps consistently halt or consistently reboot for these three cases and allow this to be configurable?

It is not clear to me why an error returned from a function results in a reboot, whereas a Hardfault does not. They both indicate a fault in the SPE.

At the very least the behaviour should be configurable, which this PR is a step towards: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/13839