e.g. in CCID mode we need to treat class-specific control requests,
and we want to do this in a way how the CCID code doesn't need to
understand about DFU.
We cannoy simply use the DFU runtime descriptor of the DFU mode, but we
have to use the descriptor of the specific currently-selected runtime
configuration. Let's iterate over the descriptors of a configuration
and find the DFU runtime descriptor in it.
We need to refer to g_dfu as a pointer from all code. In DFU mode, it
gets assigned to the address of _g_dfu, which is placed by the linker
script at the start of RAM. In runtime mode, the pointer is statically
defined to point at the start of RAM. The linker script for the runtime
(dfu environment) is adjusted to save the first 32 bytes for the _g_dfu
structure.
* we now actually route the EP0 control requests in DFU mode to the
correct handler (weak linker symbols are tricky)
* we now actually call code to read/write data from/to RAM/FLASH
