Fix doxygen

[RuRo]

This PR is rebased on top of master and assumes, that you would rebase your WebUSB branch on top of 15bc1eeb6 (master sha at the moment) before reviewing/accepting it. (The conflict with master in LUFA/Drivers/USB/Class/Common/HIDReportData.h is pretty trivial, you just need to change //@} to /**@}*/)

0. The "make -C Maintenance upgrade-doxygen (WebUSB Demo)" commit updates the doxyfile to match the changes made in abcminiuser@6c9c7d5

1. The reason, why Doxygen couldn't find the reference to BOS_DESCRIPTOR is that the documentation for the Demos is completely separate from the Core library documentation. I don't think there is an easy way to cross-reference separate documentation sources in doxygen and other Demos don't seem to use \ref when referring to library types. So I decided to just remove this \ref.

2. The reason, why Doxygen couldn't find the reference to WebUSB_URL_Descriptor_t is because it didn't expand the ATTR_PACKED macro, so it thought that the type name is ATTR_PACKED, not WebUSB_URL_Descriptor_t. I've updated the doxyfile option PREDEFINED so that the macro gets expanded.

3. There was also an issue with the docs for the code you changed in LUFA/Drivers/USB/Core/StdDescriptors.h. I've added \code/\endcode markers there. Otherwise, doxygen thinks that this is plain text and "#define" is a reference (which doesn't exist).