Kconfig: Document new def_{int,hex,string} extensions

These work like def_bool, setting the type and adding a default at the
same time. This extension fixes some Kconfig inconsistency.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

doc/porting/board_porting.rst

@@ -406,6 +406,9 @@ The following Kconfig extensions are available:
 
       orsource "Kconfig[12]"
 
+- ``def_int``, ``def_hex``, and ``def_string`` keywords, which are analogous to
+  ``def_bool``. These set the type and add a ``default`` at the same time.
+
 Old Zephyr Kconfig behavior for defaults
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