efi_loader: fix uefi secure boot with intermediate certs

efi_loader: fix uefi secure boot with intermediate certs

The general rule of accepting or rejecting an image is
 1. Is the sha256 of the image in dbx
 2. Is the image signed with a certificate that's found in db and
    not in dbx
 3. The image carries a cert which is signed by a cert in db (and
    not in dbx) and the image can be verified against the former
 4. Is the sha256 of the image in db

For example SHIM is signed by "CN=Microsoft Windows UEFI Driver Publisher",
which is issued by "CN=Microsoft Corporation UEFI CA 2011", which in it's
turn is issued by "CN=Microsoft Corporation Third Party Marketplace Root".
The latter is a self-signed CA certificate and with our current implementation
allows shim to execute if we insert it in db.

However it's the CA cert in the middle of the chain which usually ends up
in the system's db.  pkcs7_verify_one() might or might not return the root
certificate for a given chain.  But when verifying executables in UEFI,  the
trust anchor can be in the middle of the chain, as long as that certificate
is present in db.  Currently we only allow this check on self-signed
certificates,  so let's remove that check and allow all certs to try a
match an entry in db.

Open questions:
- Does this break any aspect of variable authentication since
  efi_signature_verify() is used on those as well?

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>