snippets: Add slot1-partition snippet

Adds a snippet which sets the chosen code partition to
slot1_partition, which can be used with things like MCUboot

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

snippets/slot1-partition/README.rst

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+.. _snippet-slot1-partition:
+
+Slot1 partition snippet (slot1-partition)
+#########################################
+
+.. code-block:: console
+
+   west build -S slot1-partition [...]
+
+Overview
+********
+
+This snippet changes the chosen flash partition to be the ``slot1_partition`` node, this can be
+used to build for the alternate slot in MCUboot direct-xip or firmware loader modes.
+
+Requirements
+************
+
+Partition mapping correct setup in devicetree, for example:
+
+.. literalinclude:: ../../dts/vendor/nordic/nrf52840_partition.dtsi
+   :language: dts
+   :dedent:
+   :lines: 15-

snippets/slot1-partition/slot1-partition.overlay

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+/ {
+	chosen {
+		zephyr,code-partition = &slot1_partition;
+	};
+};

snippets/slot1-partition/snippet.yml

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+name: slot1-partition
+append:
+  EXTRA_DTC_OVERLAY_FILE: slot1-partition.overlay