Hi, Raef,
The combination that worked was the most-recent commit and gnu tools (-G option). Using VS it fails at lib/ext/CMSIS_5/CMakeLists.txt:25 for lack of CMSIS RTX static libraries.
...this command worked.... $ cmake -S . -B cmake_build -DTFM_PLATFORM=mps2/an521 -DCMAKE_TOOLCHAIN_FILE=toolchain_GNUARM.cmake -G"Unix Makefiles" -- The C compiler identification is GNU 9.3.1 -- The ASM compiler identification is GNU -- Found assembler: C:/Program Files (x86)/GNU Arm Embedded Toolchain/9 2020-q2-update/bin/arm-none-eabi-gcc.exe -- Detecting C compiler ABI info -- Detecting C compiler ABI info - done -- Check for working C compiler: C:/Program Files (x86)/GNU Arm Embedded Toolchain/9 2020-q2-update/bin/arm-none-eabi-gcc.exe - skipped -- Detecting C compile features -- Detecting C compile features - done