Add encrypted file support

Add encrypted file support

In certain cases, such as network booting over an untrusted connection,
it may be useful to fully encrypt and sign the kernel files.

Enable fully encrypted boot using builtin keyring via the addition of
the string "ENCRYPTED" to the first line of the /etc/pb-lockdown file.
This disables detached (plaintext) signature verification.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>