sign_uefi: Update handling for presigned crdyboot builds

As of https://crrev.com/c/7239307, presigned crdyboot files are
installed to `efi/boot/presigned` instead of replacing the unsigned
files. Update the signer code to overwrite the unsigned files if the
presigned directory is present.

BRANCH=None
BUG=b:459486913
TEST=make runtests
TEST=scripts/image_signing/sign_official_build.sh recovery \
TEST=    disk.unsigned tests/devkeys disk.signed

Change-Id: I42418d5fe36218e562a301e279239a05d9c07cb9
Signed-off-by: Nicholas Bishop <nicholasbishop@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7239189
Reviewed-by: Benjamin Shai <bshai@google.com>
Commit-Queue: Benjamin Shai <bshai@google.com>