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This commit adds support to stm32's mboot for signe, encrypted and
compressed DFU updates.  It is based on inital work done by Andrew Leech.

The feature is enabled by setting MBOOT_ENABLE_PACKING to 1 in the board's
mpconfigboard.mk file, and by providing a header file in the board folder
(usually called mboot_keys.h) with a set of signing and encryption keys
(which can be generated by mboot_pack_dfu.py).  The signing and encryption
is provided by libhydrogen.  Compression is provided by uzlib.  Enabling
packing costs about 3k of flash.

The included mboot_pack_dfu.py script converts a .dfu file to a .pack.dfu
file which can be subsequently deployed to a board with mboot in packing
mode.  This .pack.dfu file is created as follows:
- the firmware from the original .dfu is split into chunks (so the
  decryption can fit in RAM)
- each chunk is compressed, encrypted, a header added, then signed
- a special final chunk is added with a signature of the entire firmware
- all chunks are concatenated to make the final .pack.dfu file

The .pack.dfu file can be deployed over USB or from the internal filesystem
on the device (if MBOOT_FSLOAD is enabled).

See #5267 and #5309 for additional discussion.
Signed-off-by: Damien George <damien@micropython.org>