Unfortunately, the oppo-a51f has been unmaintained for quite some time
now. The original author (harutora) has disappeared and no one else
seems to have this device. The device port has several open problems:

  1. The device was never added properly to upstream lk2nd, because
     of a special (OPPO-specific) boot image format. Users need to
     compile and install some outdated version of lk2nd, which is no
     longer (fully) compatible with newer kernel versions.

  2. The authorship of the Linux changes are unclear: They were created
     with a "no-reply" email address and without Signed-off-by line
     that clarifies the licensing of the changes and permits upstreaming
     the patches.

Since oppo-a51f is the last remaining device port in linux-postmarketos
-qcom-msm8916 with this problem, I decided to drop the patches from the
kernel, at least until harutora returns or can clarify the authorship
of the created patches.