add titles to all documents

Otherwise Sphinx is not happy when building the html target:

WARNING: toctree contains reference to document 'license' that doesn't have a title: no link will be generated
WARNING: toctree contains reference to document 'acknowledgements' that doesn't have a title: no link will be generated
WARNING: toctree contains reference to document 'revhistory' that doesn't have a title: no link will be generated
WARNING: toctree contains reference to document 'references' that doesn't have a title: no link will be generated

and it breaks the output, though surprisingly it worked fine with
singlehtml (still with the warnings).

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

Author: QSchulz

source/acknowledgements.rst

@@ -1,6 +1,7 @@
 .. SPDX-License-Identifier: Apache-2.0
 
-**Acknowledgements**
+Acknowledgements
+================
 
 FIT (Flattened Image Tree) was developed in 2008 by Marian Balakowicz and
 Bartlomiej Sieka of :index:`Semihalf`, under the guidance of Wolfgang Denk,

source/license.rst

@@ -1,6 +1,7 @@
 .. SPDX-License-Identifier: Apache-2.0
 
-**Copyright**
+Copyright
+=========
 
 | Copyright 2008 Marian Balakowicz <[email protected]>
 | Copyright 2008 Bartlomiej Sieka <[email protected]>
@@ -23,7 +24,8 @@ provision, should be addressed to:
 | Sunnyvale, CA 94085
 | United States
 
-**License Information**
+License Information
+-------------------
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

source/references.rst

@@ -1,5 +1,8 @@
 .. SPDX-License-Identifier: Apache-2.0
 
+References
+==========
+
 .. [dtspec] Devicetree Specification
    https://www.devicetree.org/specifications
 .. [VBE] Verified Boot for Embedded (VBE)

source/revhistory.rst

@@ -1,6 +1,7 @@
 .. SPDX-License-Identifier: Apache-2.0
 
-**Revision History**
+Revision History
+================
 
 .. _revision-history:
 .. tabularcolumns:: | p{1.5cm} p{2.25cm} p{11.50cm} |