binman: Add a command to generate bintool docs

Each bintool has some documentation which can be useful for the user.
Add a new command that collects this and writes it into a .rst file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/cmdline.py b/tools/binman/cmdline.py
index 5ccb238..0626b85 100644
--- a/tools/binman/cmdline.py
+++ b/tools/binman/cmdline.py
@@ -130,6 +130,9 @@
         help='Update an ELF file with the output dtb: infile,outfile,begin_sym,end_sym')
 
     subparsers.add_parser(
+        'bintool-docs', help='Write out bintool documentation (see bintool.rst)')
+
+    subparsers.add_parser(
         'entry-docs', help='Write out entry documentation (see entries.rst)')
 
     list_parser = subparsers.add_parser('ls', help='List files in an image')