sm8550-hdk: Add instructions to boot from the on-board UFS

Change-Id: I33e5a66d13bd3d3341af3059afa89fb70fd57c46
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
diff --git a/docs/devices/sm8550-hdk.rst b/docs/devices/sm8550-hdk.rst
index 9cbb4a2..102e7c5 100644
--- a/docs/devices/sm8550-hdk.rst
+++ b/docs/devices/sm8550-hdk.rst
@@ -122,6 +122,10 @@
 
 It should boot sm8x50 devices to UI with software rendering support.
 
+.. note::
+   To boot from the on-board UFS, build AOSP images with ``TARGET_SDCARD_BOOT=false``
+   build flag and follow the same flashing and booting instructions as above.
+
 
 Building and booting with custom kernel
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