As part of r336741, BOOTFILES needs special handling when
cross building on stable/11, where the path should be:

/usr/obj/TARGET.TARGET_ARCH/[...]

The change in r336741, which was a direct commit to stable/11
was not complete enough to evaluate where BOOTFILES should be.

This, as was r336741, is a direct commit to stable/11.

Tested with: stable/11 BANANAPI build
Sponsored by: The FreeBSD Foundation