build: cleanup possibly dangling Python 3 host symlink

When switching from master branch to 19.07 or older, we need to ensure
that Python symlink in staging bin directory points to Python 2.

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.

This is a cherry-pick of 3b68fb57c9
with python3 instead of python2

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
(cherry picked from commit 799de7c641)

include/prereq-build.mk

@@ -141,6 +141,8 @@ $(eval $(call SetupHostCommand,wget,Please install GNU 'wget', \
 $(eval $(call SetupHostCommand,perl,Please install Perl 5.x, \
 	perl --version | grep "perl.*v5"))
 
+$(eval $(call CleanupPython3))
+
 $(eval $(call SetupHostCommand,python,Please install Python 2.x, \
 	python2.7 -V 2>&1 | grep 'Python 2.7', \
 	python2 -V 2>&1 | grep 'Python 2', \

include/prereq.mk

@@ -66,6 +66,18 @@ define RequireHeader
   $$(eval $$(call Require,$(1),$(2)))
 endef
 
+define CleanupPython3
+  define Require/python3-cleanup
+	if [ -f "$(STAGING_DIR_HOST)/bin/python" ] && \
+		$(STAGING_DIR_HOST)/bin/python -V 2>&1 | \
+		grep -q 'Python 3'; then \
+			rm $(STAGING_DIR_HOST)/bin/python; \
+	fi
+  endef
+
+  $$(eval $$(call Require,python3-cleanup))
+endef
+
 define QuoteHostCommand
 '$(subst ','"'"',$(strip $(1)))'
 endef