Prepare for handling pkg state with files. So it requires having the
path defined before declaring targets. In addition, it cleans up the
old git-download target.
In this Makefile, 'CURDIR' is RIOTBASE/pkg/tiny-asn1 directory which does not
exist and not PKGDIRBASE/tiny-asn1.
The correct include directory is set by pkg/tiny-asn1/Makefile.include.