this adds the DFU as application, allowing to flash the bootloader.
a USB DFU alternative is added to flash the bootloader partition.
when the DFU is started as bootloader, the partition/alternative
to flash the bootloader is marked as "not available", and
ineffective.
the same happens for the application partition when DFU is started
as application.
this distinction is make at compile time, not at runtime, because
of size restrictions (the bootloader was already close to the
16 kB limit).
*_dfu_flash.bin should not be mixed with *_dfu_dfu.bin.
*_dfu_dfu.bin should be flashed as application using the already
existing DFU bootloader.
once this images is started (as application), the *_dfu_flash.bin
should be flashed as bootloader using the DFU application.
once the DFU bootloader has been flashed, soft resetting
(not re-powering) will cause the bootloader to start, allowing to
flash the application with a normal image (e.g. not DFU),
replacing the DFU application.
this switch to DFU only happens after downloading (e.g. flashing).
it is planned to have the DFU application erase itself after
flashing, but this is currently not implemented.
Change-Id: Ic273bb593a7669111b0219fe301d7897419167c8
as for the main application firmware, the DFU bootloader firmware
now also has the unique chip ID as iSerial in the USB description,
and an additional empty USB configuration indicates the firmware
version (e.g. DFU bootloader version).
these are only visible when the device is in DFU mode.
Change-Id: I11a2cd8079fda374d816da180f39f1c33d10af60
