[crypto] Start up RBG on demand if neededrbgstart

The ANS X9.82 specification implicitly assumes that the RBG_Startup
function will be called before it is needed, and includes checks to
make sure that Generate_function fails if this has not happened.
However, there is no well-defined point at which the RBG_Startup
function is to be called: it's just assumed that this happens as part
of system startup.

We currently call RBG_Startup to instantiate the DRBG as an iPXE
startup function, with the corresponding shutdown function
uninstantiating the DRBG.  This works for most use cases, and avoids
an otherwise unexpected user-visible delay when a caller first
attempts to use the DRBG (e.g. by attempting an HTTPS download).

The download of autoexec.ipxe for UEFI is triggered by the EFI root
bus probe in efi_probe().  Both the root bus probe and the RBG startup
function run at STARTUP_NORMAL, so there is no defined ordering
between them.  If the base URI for autoexec.ipxe uses HTTPS, then this
may cause random bits to be requested before the RBG has been started.

Extend the logic in rbg_generate() to automatically start up the RBG
if startup has not already been attempted.  If startup fails
(e.g. because the entropy source is broken), then do not automatically
retry since this could result in extremely long delays waiting for
entropy that will never arrive.

Reported-by: Michael Niehaus <niehaus@live.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

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