pkg-infra: fix missing 3993 for AUTOTARGET's SUBDIR

Ensure <pkg>_SUBDIR gets resolved late, so that E.G. the default values
for host packages are taken into consideration.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>