python,python3: add host python pip install support

To install Python packages host side, that
may be needed for a build.

The intent, is to try to reduce host-side Python
packages being installed via LEDE/OpenWrt build system.
Because those seem like a pain to maintain.

The idea is adapted from Yousong's `python-packages`
package.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python3/files/python3-package.mk

@@ -126,6 +126,9 @@ define Build/Compile/Py3Mod
 endef
 
 define Py3Build/Compile/Default
+	$(foreach pkg,$(HOST_PYTHON3_PACKAGE_BUILD_DEPENDS),
+		$(call host_python3_pip_install_host,$(pkg))
+	)
 	$(call Build/Compile/Py3Mod,, \
 		install --prefix="/usr" --root="$(PKG_INSTALL_DIR)" \
 		--single-version-externally-managed \