TF-M: Use FetchContent for fetching

Previously, we used `ExternalProject` to both fetch and build TF-M, and
fetching happened at build time. Now we switch to `FetchContent` for the
fetching step, because
* it fetches code during configuration instead of build, and no time is
wasted contacting the git server on every rebuild
* a local copy of a component can be used, saving time and disk space
and allowing a modified version to be used

To use a local copy of TF-M, pass

    -D FETCHCONTENT_SOURCE_DIR_TF-M=<path-to-local-TF-M>

This has been added to the documentation.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>