release: Don't install ubldr.bin

ubldr.bin was obsoleted by our uboot ports last year, so this is
completely unused in the default config (some customers still use
it, but that's not relevant to this script). Don't copy it at all
since it won't be used for re@ produced images.

Sponsored by: Netflix
Reviewed by: kevans, gjb (re@)
Differential Revision: https://reviews.freebsd.org/D34192