package/python-iptables: fix _find_library()

While the commit [1] already fixed some runtime issue with
uClibc toolchain, the same test TestPythonPy3Iptables fail
with Glibc toolchain.

  xtables_version = 12
  if xtables_version:
      _searchlib = "libxtables.so.%s" % (xtables_version,)
  else:
      _searchlib = "xtables"
  _lib_xtables, xtables_version = find_library(_searchlib)

  _lib_xtables and xtables_version are null with glibc

The implementation of find_library() rely on the custom
_find_library() that try to use the IPTABLES_LIBDIR
environment variable (that does not exist in the context
of Buildroot).

Within the scope of buildroot we can determine what
IPTABLES_LIBDIR should be at build time and replace the
calls to os.environ.get('IPTABLES_LIBDIR', None) with
the correct value.

[1] d341ec035067120b456f669a826e26eb594c0237

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/python-iptables/python-iptables.mk

@@ -15,6 +15,7 @@ define PYTHON_IPTABLES_SET_XTABLES_ENV_VARS
 	XTABLES_VERSION=`awk '/XTABLES_VERSION_CODE/ {print $$NF}' $(STAGING_DIR)/usr/include/xtables-version.h`; \
 	sed -i "s%os.getenv(\"PYTHON_IPTABLES_XTABLES_VERSION\")%$$XTABLES_VERSION%" $(@D)/iptc/xtables.py
 	sed -i "s%os.getenv(\"XTABLES_LIBDIR\")%\"/usr/lib/xtables\"%" $(@D)/iptc/xtables.py
+	sed -i "s%os.environ.get('IPTABLES_LIBDIR', None)%\"/usr/lib\"%" $(@D)/iptc/util.py
 endef
 
 PYTHON_IPTABLES_PRE_BUILD_HOOKS += PYTHON_IPTABLES_SET_XTABLES_ENV_VARS