Build: Support out-of-tree platforms

Allows -DTFM_PLATFORM to be set to paths outside the TFM tree.

e.g.
cmake ... -DTFM_PLATFORM=../../../../tfm_platforms/my_platform

Note: TFM_PLATFORM currently only accepts paths relative from
platform/ext/target

Change-Id: I99d194fdee43ffa29da67b22eb79ef5e450c1559
Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
4 files changed