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>

devel/asu/Makefile

@@ -6,7 +6,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=asu
 PKG_VERSION:=0.2.3
-PKG_RELEASE=2
+PKG_RELEASE:=3
 
 PKG_LICENSE:=GPL-3.0
 
@@ -37,7 +37,6 @@ define Package/asu
              +gunicorn3 +python3-openssl +python3-pyodbc +python3-yaml \
              +libustream-mbedtls +ca-certificates +gnupg
     USERID:=asu:asu
-    VARIANT:=python3
 endef
 
 define Package/asu/description

devel/meson/Makefile

@@ -2,7 +2,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=meson
 PKG_VERSION:=0.54.0
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PYPI_NAME:=meson
 PKG_HASH:=dde5726d778112acbd4a67bb3633ab2ee75d33d1e879a6283a7b4a44c3363c27
@@ -40,7 +40,6 @@ define Package/meson
   TITLE:=meson
   URL:=https://mesonbuild.com/
   DEPENDS:=+ninja +python3-pkg-resources
-  VARIANT:=python3
 endef
 
 define Package/meson/description