trusted-firmware-m: rename AN521 target to BOARD_MPS2_AN521_CPU0_NS

Since we now have separate Kconfig symbols for the different
MPS2 AN521 board variants, we need to update the BOARD switch
for the TF-M target on MPS2 AN521.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
1 file changed