cmake.bbclass: Make sure EXTRA_OECMAKE has a sane default

Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>

meta/classes/cmake.bbclass

@@ -3,6 +3,8 @@ DEPENDS += " cmake-native "
 # We want the staging and installing functions from autotools
 inherit autotools
 
+EXTRA_OECMAKE ?= ""
+
 cmake_do_configure() {
      cmake . -DCMAKE_INSTALL_PREFIX:PATH=${prefix} -Wno-dev \
      -DCMAKE_FIND_ROOT_PATH=${STAGING_DIR_HOST} ${EXTRA_OECMAKE}