package/pkg-waf.mk: drop bundled waf/waflib when external waf is used

Waf requires that the version of the waf script matches the version of
waflib, so drop any bundled waf/waflib if _NEEDS_EXTERNAL_WAF is used, as
otherwise waf errors out with errors like:

Waf script '2.0.24' and library '1.9.3' do not match

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
[Peter: Run as a post-patch hook as suggested by Yann]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-waf.mk

@@ -38,6 +38,13 @@ WAF_OPTS = $(if $(VERBOSE),-v) -j $(PARALLEL_JOBS)
 
 define inner-waf-package
 
+# The version of waflib has to match with the version of waf,
+# otherwise waf errors out with:
+# Waf script 'X' and library 'Y' do not match
+define WAF_PACKAGE_REMOVE_WAF_LIB
+	$(RM) -fr $(@D)/waf $(@D)/waflib
+endef
+
 # We need host-python3 to run waf
 $(2)_DEPENDENCIES += host-python3
 
@@ -47,6 +54,7 @@ $(2)_NEEDS_EXTERNAL_WAF ?= NO
 ifeq ($$($(2)_NEEDS_EXTERNAL_WAF),YES)
 $(2)_DEPENDENCIES += host-waf
 $(2)_WAF = $$(HOST_DIR)/bin/waf
+$(2)_POST_PATCH_HOOKS += WAF_PACKAGE_REMOVE_WAF_LIB
 else
 $(2)_WAF ?= ./waf
 endif