Build: Support passing absolute paths to TFM_PLATFORM

For convenience when using out-of-tree platforms.

Change-Id: I6783eabe9e13c0bdc2412cebd1ff25277cd5dda0
Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
diff --git a/docs/getting_started/tfm_build_instruction.rst b/docs/getting_started/tfm_build_instruction.rst
index 1a8e4b9..5576327 100644
--- a/docs/getting_started/tfm_build_instruction.rst
+++ b/docs/getting_started/tfm_build_instruction.rst
@@ -69,7 +69,7 @@
 | Parameter            | Description                                           |
 +======================+=======================================================+
 | TFM_PLATFORM         | The target platform as a path from the base directory |
-|                      | ``/platform/ext/target``                              |
+|                      | ``/platform/ext/target``, or as an absolute path.     |
 +----------------------+-------------------------------------------------------+
 | TFM_TOOLCHAIN_FILE   | The path to the toolchain file that corresponds to    |
 |                      | the desired compiler.                                 |