[TF-M] -DBL2=False causes duplicates in ASM_FLAGS in makefiles

For some reason that I've not been able to track down, using -DBL2=False on the cmake command line causes ASM_FLAGS to have duplicated debug flags and defines.

It does not happen to C_FLAGS and it doesn't happen with -DBL2=True.

I've worked around the issue in https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/5365, but a cleaner solution would be to avoid this in the first place.

I would appreciate if someone with more cmake experience could have a go at this.

It causes build failures with IAR, and ARMCLANG and GNUARM doesn't care about the duplicates.

Thomas