Makefile.legacy: fix recursive invocation with BUILDROOT_DL_DIR and _CONFIG

The legacy support for the old BUILDROOT_DL_DIR and BUILDROOT_CONFIG
breaks down when make is invoked recursively - which is done in a few
cases, e.g. silentoldconfig, external-defs, ... These targets always
give a legacy error.

For BUILDROOT_DL_DIR, this is fixed by making sure that the original
value of BR2_DL_DIR taken from the environment is also exported again.

For BUILDROOT_CONFIG, this is fixed with an additional comparison of
the environment variable's value with the fake value that we introduce
ourselves.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>