buildroot

waf: use HOST_WAF_DL_DIR instead of WAF_DL_DIR

The infrastructure only provides HOST_WAF_DL_DIR, because this package
is host only. Ideally the infra should provide WAF_DL_DIR, but it
doesn't currently, and that requires more significant changes.

Fixes:

  http://autobuild.buildroot.org/results/01d8c21df4baa9ecc6a0e45744fe6a4d086a0652/

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>