lib/security: hard_lockdown flag to stop runtime disable of signed boot

lib/security: hard_lockdown flag to stop runtime disable of signed boot

Currently if signed-boot is enabled in configure the presence of the
LOCKDOWN_FILE is used as a runtime determination to perform the actual
verification.  In some environments this may be acceptable or even the
intended operation but in other environments could be a security hole
since the removal of the file will then cause boot task verification.
Add a 'hard_lockdown' enable flag to generate a HARD_LOCKDOWN
preprocessor definition to force the system to always do a signed boot
verification for each boot task, which in the case of a missing file the
boot will fail.

Signed-off-by: Brett Grandbois <brett.grandbois@opengear.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>