Export variables in the arm/*.conf files because they
need to be passed through release.sh to arm/release.sh.

Set MK_TESTS=no for the xdev target, since that blows
up impressively.

This commit gets the arm build back to the state that
it was in before the XDEV/XDEV_ARCH deprecation, in that
the builds still fail, but they fail in the same way they
did before.

Sponsored by: The FreeBSD Foundation