refactor(measured-boot): move image identifier strings to a common header

The image identifier strings are used across different measured boot
backends. Move them to a common location to avoid the redefiniton
of these per backend and to avoid code duplication.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: I58897b9a19396be932ca5d230ee00858c09ef03f
2 files changed