Docs: Add missing platforms to documentation

Add missing documentation for Musca-S1 and AN539

Change-Id: Ic98d3a60c3ea89513a0b5b8cbf6ad6e117506162
Signed-off-by: Marton Berke <marton.berke@arm.com>
diff --git a/docs/user_guides/tfm_user_guide.rst b/docs/user_guides/tfm_user_guide.rst
index 1c770e9..a25894b 100644
--- a/docs/user_guides/tfm_user_guide.rst
+++ b/docs/user_guides/tfm_user_guide.rst
@@ -243,6 +243,16 @@
 
             srec_cat bl2/ext/mcuboot/mcuboot.bin -Binary -offset 0xA000000 tfm_sign.bin -Binary -offset 0xA020000 -o tfm.hex -Intel
 
+    - For Musca-S1
+
+        - Windows::
+
+            srec_cat.exe bl2\ext\mcuboot\mcuboot.bin -Binary -offset 0xA000000 tfm_sign.bin -Binary -offset 0xA020000 -o tfm.hex -Intel
+
+        - Linux::
+
+            srec_cat bl2/ext/mcuboot/mcuboot.bin -Binary -offset 0xA000000 tfm_sign.bin -Binary -offset 0xA020000 -o tfm.hex -Intel
+
 #. Power up the Musca board by connecting it to a computer with a USB lead.
    Press the ``PBON`` button if the green ``ON`` LED does not immediately turn
    on. The board should appear as a USB drive.