Add more const-ness to the stream endpoint/pipe functions where possible.

Bootloaders/HID/Doxygen.conf

@@ -615,7 +615,8 @@ RECURSIVE              = YES
 # excluded from the INPUT source files. This way you can easily exclude a
 # subdirectory from a directory tree whose root is specified with the INPUT tag.
 
-EXCLUDE                = Documentation/
+EXCLUDE                = Documentation/ \
+                         HostLoaderApp/
 
 # The EXCLUDE_SYMLINKS tag can be used select whether or not files or
 # directories that are symbolic links (a Unix filesystem feature) are excluded