doxygen: fix issues with recent Doxygen versions

This fixes parsing of several documentation files which do not have
dot in the filename.

style.txt change fixes doxygen warnings and ensures proper display of
comment begin/end instructions in HTML.

Tested with Doxygen 1.8.13.

Change-Id: I9fd9ac3b51b47076b8fdbd3e93b90c3eba9b9631
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/4342
Tested-by: jenkins

Doxyfile.in

@@ -216,7 +216,7 @@ OPTIMIZE_OUTPUT_VHDL   = NO
 # .inc files as Fortran files (default is PHP), and .f files as C (default is Fortran), 
 # use: inc=Fortran f=C
 
-EXTENSION_MAPPING      = 
+EXTENSION_MAPPING      = no_extension=C
 
 # If you use STL classes (i.e. std::string, std::vector, etc.) but do not want
 # to include (a tag file for) the STL sources as input, then you should

doc/manual/style.txt

@@ -176,10 +176,10 @@ comments.
  * @returns The value(s) returned, or possible error conditions.
  */
 @endverbatim
-  -# The block should start on the line following the opening @c /**.
-  -# The end of the block, \f$*/\f$, should also be on its own line.
+  -# The block should start on the line following the opening @c /\**.
+  -# The end of the block, @c *‍/, should also be on its own line.
   -# Every line in the block should have a @c '*' in-line with its start:
-    - A leading space is required to align the @c '*' with the @c /** line.
+    - A leading space is required to align the @c '*' with the @c /\** line.
     - A single "empty" line should separate the function documentation
       from the block of parameter and return value descriptions.
     - Except to separate paragraphs of documentation, other extra