treewide: Remove Python variants for non-Python packages

This removes Python-related build variants, and adds
PYTHON3_PKG_BUILD:=0 and minor build adjustments (where appropriate),
for non-Python packages. There should be no changes to build output.

This also updates some include paths for python3-package.mk and/or
python3-host.mk to be relative to the package Makefile.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

utils/bigclown/bigclown-control-tool/Makefile

@@ -6,7 +6,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=bigclown-control-tool
 PKG_VERSION:=0.2.0
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PYPI_NAME:=bch
 PKG_HASH:=4cd73b92757fce7275a4744baed411c867af2e671c521b90d6690b2320851d58
@@ -32,7 +32,6 @@ define Package/bigclown-control-tool
     +python3-pyserial \
     +python3-yaml \
     +python3-simplejson
-  VARIANT:=python3
 endef
 
 $(eval $(call Py3Package,bigclown-control-tool))