[crypto] Allow for multiple cross-signed certificate download attempts

[crypto] Allow for multiple cross-signed certificate download attempts

Certificates issued by Let's Encrypt have two options for their chain
of trust: the chain can either terminate in the self-signed ISRG Root
X1 root certificate, or in an intermediate ISRG Root X1 certificate
that is signed in turn by the self-signed DST Root CA X3 root
certificate.  This is a historical artifact: when Let's Encrypt first
launched as a project, the chain ending in DST Root CA X3 was used
since existing clients would not have recognised the ISRG Root X1
certificate as a trusted root certificate.

The DST Root CA X3 certificate expired in September 2021, and so is no
longer trusted by clients (such as iPXE) that validate the expiry
times of all certificates in the certificate chain.

In order to maintain usability of certificates on older Android
devices, the default certificate chain provided by Let's Encrypt still
terminates in DST Root CA X3, even though that certificate has now
expired.  On newer devices which include ISRG Root X1 as a trusted
root certificate, the intermediate version of ISRG Root X1 in the
certificate chain is ignored and validation is performed as though the
chain had terminated in the self-signed ISRG Root X1 root certificate.
On older Android devices which do not include ISRG Root X1 as a
trusted root certificate, the validation succeeds since Android
chooses to ignore expiry times for root certificates and so continues
to trust the DST Root CA X3 root certificate.

This backwards compatibility hack unfortunately breaks the cross-
signing mechanism used by iPXE, which assumes that the certificate
chain will always terminate in a non-expired root certificate.

Generalise the validator's cross-signed certificate download mechanism
to walk up the certificate chain in the event of a failure, attempting
to find a replacement cross-signed certificate chain starting from the
next level up.  This allows the validator to step over the expired
(and hence invalidatable) DST Root CA X3 certificate, and instead
download the cross-signed version of the ISRG Root X1 certificate.

This generalisation also gives us the ability to handle servers that
provide a full certificate chain including their root certificate:
iPXE will step over the untrusted public root certificate and attempt
to find a cross-signed version of it instead.

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