package/pkg-waf.mk: allow for override of <pkg>_WAF directive

Some packages have their own waf copy, but not located at the
top-level directory. In order to support those packages, we allow
packages to override the default <pkg>_WAF location.

Signed-off-by: David GOUARIN <dgouarin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>