x86: add a tboot Kconfig option

x86: add a tboot Kconfig option

Allows for the conditional inclusion of tboot related functionality
via Kconfig

The default configuration for the new CONFIG_TBOOT option is 'y', so the
behavior out of the box remains unchanged.  The addition of the option allows
advanced users to disable system behaviors associated with tboot at compile
time rather than relying on the run-time detection and configuration.

The CONFIG_CRYPTO option is 'n' by default and selected by the individual users
that require the functionality.  Currently, the only user is tboot.

Signed-off-by: Derek Straka <derek@asterius.io>
Acked-by: Jan Beulich <jbeulich@suse.com>