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>
diff --git a/package/pkg-waf.mk b/package/pkg-waf.mk
index 35e56d3..038d894 100644
--- a/package/pkg-waf.mk
+++ b/package/pkg-waf.mk
@@ -46,7 +46,7 @@
 $(2)_DEPENDENCIES += host-waf
 $(2)_WAF = $$(HOST_DIR)/bin/waf
 else
-$(2)_WAF = ./waf
+$(2)_WAF ?= ./waf
 endif
 
 $(2)_BUILD_OPTS				?=