python-efl: fix do_configure_prepend to be reentrant

* without '=' it added ${STAGING_DIR_HOST} prefix again when
  do_configure was executed twice in the same WORKDIR
  causing issues when looking for header files in python-edje,
  python-ecore, python-elementary and python-emotion
  ./python-edje/configure.ac:CYTHON_EDJE_INCLUDEDIR=`$PKG_CONFIG --variable=includedir "python-evas >= ${python_evas_version}"`
  ./python-ecore/configure.ac:CYTHON_ECORE_EVAS_INCLUDEDIR=""
  ./python-ecore/configure.ac:CYTHON_ECORE_EVAS_INCLUDEDIR=`$PKG_CONFIG --variable=includedir "python-evas >= ${python_evas_version}"`
  ./python-elementary/configure.ac:CYTHON_ELEMENTARY_INCLUDEDIR=`$PKG_CONFIG --variable=includedir "python-evas >= ${python_evas_version}"`
  ./python-emotion/configure.ac:CYTHON_EMOTION_INCLUDEDIR=`$PKG_CONFIG --variable=includedir "python-evas >= ${python_evas_version}"`

* config.log diffs confirms duplicated ${STAGING_DIR_HOST} prefix
  < CYTHON_ECORE_EVAS_INCLUDEDIR='/OE/jansa-test/shr-core/tmp-eglibc/sysroots/tuna/usr/include/python-evas'
  ---
  > CYTHON_ECORE_EVAS_INCLUDEDIR='/OE/jansa-test/shr-core/tmp-eglibc/sysroots/tuna/OE/jansa-test/shr-core/tmp-eglibc/sysroots/tuna/usr/include/python-evas'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>