feat(docs): update mboot threat model

Restructure Measured Boot threat model for more description and clarity:
- Add what critical assets are to be protected.
- Mention other attributes and the possible attacks.
- Rephrase the section that describes the Measured Boot backends.

Change-Id: I6577a56184992bf16f4aa1b773d1636781cbb049
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Signed-off-by: Abhi Singh <abhi.singh@arm.com>
3 files changed