build: cleanup possibly dangling Python 2 host symlink

When bumping buildroot to Python 3, we need to assure, that Python
symlink in staging bin directory points to Python >= 3.5 as well.

We can't rely completly just on SetupHostCommand as its executed only in
cases when the $(STAGING_DIR_HOST)/bin/python doesn't already exist, so
we need to remove it before running SetupHostCommand.

Acked-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

include/prereq.mk

@@ -66,6 +66,18 @@ define RequireHeader
   $$(eval $$(call Require,$(1),$(2)))
 endef
 
+define CleanupPython2
+  define Require/python2-cleanup
+	if [ -f "$(STAGING_DIR_HOST)/bin/python" ] && \
+		$(STAGING_DIR_HOST)/bin/python -V 2>&1 | \
+		grep -q 'Python 2'; then \
+			rm $(STAGING_DIR_HOST)/bin/python; \
+	fi
+  endef
+
+  $$(eval $$(call Require,python2-cleanup))
+endef
+
 define QuoteHostCommand
 '$(subst ','"'"',$(strip $(1)))'
 endef