chapter2: make /images optional subnodes its own subsection

We have a subsection for mandatory and conditionally mandatory
properties but didn't have a subsection for the option subnodes, so make
it one.

Signed-off-by: Quentin Schulz <[email protected]>

Author: QSchulz

source/chapter2-source-file-format.rst

@@ -381,7 +381,8 @@ load
     "u-boot"
         image is a U-Boot image
 
-Optional nodes:
+Optional nodes
+~~~~~~~~~~~~~~
 
 hash-1
     Each hash sub-node represents a separate hash or checksum