imgtool: Add CBOR encoded boot record to TLV area

Add new '--boot-record' option for imgtool to add a new type of TLV to
the image manifest called BOOT_RECORD. This TLV contains CBOR encoded
data with some basic information about the image (SW component) it
belongs to, these are the following:
- SW type (role of the software component)
- SW version
- Signer ID (identifies the signing authority)
- Measurement value (hash of the image)
- Measurement type (algorithm used to calculate the measurement value)

The boot_record.py file and most of the modifications in image.py are
coming from the Trusted Firmware-M project
(https://www.trustedfirmware.org/about/

).
Hash of the source commit: 08d5572b4bcee306d8cf709c2200359a22d5b72c.

This patch is based on the recommendations of Arm's Platform Security
Architecture (PSA) and its purpose is to support compliance with it.

Change-Id: I379ccc57b48ad2311837cb3fd90f5f9d1c9b5bac
Signed-off-by: David Vincze <david.vincze@linaro.org>