Change the default BR2_DL_DIR location

Change to default Download dir so that different 'make O=<directory>'
commands will use the same location by default without having to
create a symlink in <directory> or having to export BUILDROOT_DL_DIR.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>