loader/uboot: Move to foo.ldconfig

Move to the foo.ldconfig convention to match the rest of the boot
loader. No functional change intended.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44059

(cherry picked from commit 3d6239d2449ee085a9a078c378659469fd69b135)