SecureBoot

Introduction
With the advent of the T2 macOS gained the ability to verify the integrity of the OS as it is booted.

Verified Components

 * The T2 verifies MacEFI via img4 and feeds it to the Intel CPU via eSPI
 * MacEFI verifies the `boot.efi` component
 * If in Full Security mode it requires a im4m manifest that is specific to the T2 ECID
 * If in Medium Security mode it requires a im4m manifest that is specific to the T2 CPID