modules: mbedtls: Allow custom mbedtls implementation

-The current scheme in zephyr has the two choices MBEDTLS_BUILTIN
 and MBEDTLS_LIBRARY, but the choice of MBEDTLS_LIBRARY requires
 setting CONFIG_MBEDTLS_INSTALL_PATH for includes and library linking.
 This may not be neccesary when an alternative implementation of the
 library is being used. This adds support for custom choices in
 MBEDTLS_IMPLEMENTATION which can be added in an out-of-tree Kconfig
 file.
-Made else an elseif(CONFIG_MBEDTLS_LIBRARY.
-Removed reduntant assertion between the two choices.

Signed-off-by: Frank Audun Kvamtrø <frank.kvamtro@nordicsemi.no>