kern/efi/sb: Enforce verification of font files

kern/efi/sb: Enforce verification of font files

As a mitigation and hardening measure enforce verification of font
files. Then only trusted font files can be load. This will reduce the
attack surface at cost of losing the ability of end-users to customize
fonts if e.g. UEFI Secure Boot is enabled. Vendors can always customize
fonts because they have ability to pack fonts into their GRUB bundles.

This goal is achieved by:

  * Removing GRUB_FILE_TYPE_FONT from shim lock verifier's
    skip-verification list.

  * Adding GRUB_FILE_TYPE_FONT to lockdown verifier's defer-auth list,
    so font files must be verified by a verifier before they can be loaded.

Suggested-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>