boot: Define interface for data sharing with runtime

Define interface for sharing boot status (certain attributes of the
authenticated images) and adding arbitrary data in TLV encoded format
to a shared data area between the bootloader and runtime SW.

The boot_record.h file was copied (with minor modifications) from the
Trusted Firmware-M project (https://www.trustedfirmware.org/about/

).
Hash of the source commit: 08d5572b4bcee306d8cf709c2200359a22d5b72c.

Change-Id: Ia25bac27e9f1ce7faa5043c5a0455c804a24701e
Signed-off-by: David Vincze <david.vincze@arm.com>