Fix passing ${CONF_FILES} (which contains MAKE_CONF and
SRC_CONF, __MAKE_CONF and SRCCONF, respectively) through
to arm_install_base() and chroot_arm_build_release().
This prevents failures when the target image is intended
to be build with make.conf(5) and src.conf(5) overrides,
which are correctly handled for non-embedded image builds.

Reported and tested by: Daniel Engberg
PR: 238615
MFC after: 3 days
Sponsored by: The FreeBSD Foundation