Set kldxref_enable="YES" for ARM images. Without this, the images are missing the /boot/kernel/linker.hints file, which breaks loading some of the modules with dependencies, eg cfiscsi.ko. It would be better to run kldxref(8) when building the images, but at this point that tool cannot handle files for architectures other than its own.
Note that I don't really have a way to test this change, so any help here would be welcome.
This fixes https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218166.