cmake: use zephyr_library_app_memory for mem partition placement

This commit removes the hard coded mbed TLS library name
`lib..__modules__crypto__mbedtls.a` in top-level CMakeLists.txt file
and instead uses zephyr_library_app_memory function.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>