Fix variable expansion in conditional overrides

When using conditional override syntax, this should only be combined
with the append/prepend/remove operators. This commit rewrites the
claas-native and class-target overrides such that it does not replace
any recipe set values of EXTRA_OECMAKE (reading the removed syntax it is
very counter-intuitive, but sadly really happens). The problem was:

EXTRA_OECMAKE:class-target += " foo" # from class
EXTRA_OECMAKE += " baa" # from recipe

to be evaluated to EXTRA_OECMAKE = " foo". By this change, the values
are appended as expected, even if the recipe is not using conditional
overrides for the value assignment.

classes/cmake_kf5.bbclass

@@ -6,20 +6,23 @@
 
 inherit cmake_qt5
 
-EXTRA_OECMAKE:class-native += " \
+EXTRA_OECMAKE:append:class-native = " \
     -DOE_KF5_PATH_HOST_ROOT=${STAGING_DIR_HOST} \
     -DBUILD_TESTING=OFF \
     -DBUILD_DESIGNERPLUGIN=OFF \
 "
 
-EXTRA_OECMAKE:class-target += " \
+EXTRA_OECMAKE:append:class-target = " \
     -DOE_KF5_PATH_HOST_ROOT=${STAGING_DIR_HOST} \
     -DBUILD_TESTING=OFF \
     -DKF5_HOST_TOOLING=${STAGING_DIR_NATIVE}/${libdir}/cmake \
     -DBUILD_DESIGNERPLUGIN=OFF \
 "
 
-DEPENDS += "extra-cmake-modules qttools-native"
+DEPENDS:append = " \
+    extra-cmake-modules \
+    qttools-native \
+"
 
 # don't bother with translations for host tools
 do_configure:prepend:class-native() {