documentation: mention jailbreak

Documentation/how-to-build.rst

+.. _how_to_build:
+
 How To Build
 ============
 If you just want to write MicroPython code for card10, you probably **won't**
@@ -102,6 +104,7 @@ firmware features:
   info related to BLE.
 - ``-Ddebug_core1=true``: Enable the core 1 SWD lines which are exposed on the
   SAO connector.  Only use this if you have a debugger which is modified for core 1.
+- ``-Djailbreak_card10=true``: Enable execution of .elf l0dables on core 1.
 
 .. warning::
 

Documentation/overview.rst

@@ -51,6 +51,9 @@ Next to Pycardium, other bare-metal code can also run on core 1.  For example,
 a Rustcardium or C-cardium.  These l0dables must be compiled using our special
 linker script and should link against the api-caller library so they can
 interface with the :ref:`epicardium_api`.
+Note: this feature is disabled by default and has to be enabled at build time.
+To do this, run ``bootstrap.sh`` with the option ``-Djailbreak_card10=true``
+and rebuild the firmware as described in :ref:`how_to_build`.
 
 .. todo::