The intention of the existing code (as documented in its own comments)
is that it should be possible to override the list of trusted root
certificates using a "trust" setting held in non-volatile stored
options.  However, the rootcert_init() function currently executes
before any devices have been probed, and so will not be able to
retrieve any such non-volatile stored options.

Fix by executing rootcert_init() only after devices have been probed.
Since startup functions may be executed multiple times (unlike
initialisation functions), add an explicit flag to preserve the
property that rootcert_init() should run only once.

As before, if an explicit root of trust is specified at build time,
then any runtime "trust" setting will be ignored.

Signed-off-by: Michael Brown <mcb30@ipxe.org>