tfm: Fix cmake library handling
tfm: Fix cmake library handling

Make tfm_api a proper Zephyr library and add all sources to it.

Signed-off-by: Øyvind Rønningstad <5f9e1eedbdafebd99226f1498f634e1047455d88@nordicsemi.no>

galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr