buildscript: Apply variant to builddir in build step

The build variant previously only got applied in the prepare
step. Therefore selecting a new variant for a subsequent build
without calling prepare again produced a build with a different
variant than the selected one.

As the filename and version number is evaluated in the prebuild
step, which is called on every build, the version number did use
selected variant, so the build variant did not match the variant
in the version string.

As applying the variant only causes the fff-base package to be
fully rebuilt, this step is moved into a function and also executed
in buildscripts prebuild step. It is therefore always applied when
starting a new build.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

buildscript

@@ -131,10 +131,9 @@ prepare() {
 
     patch_target
 
-    # set the variant for this build
-    cp "./src/packages/fff/fff/variant-$(cat selected_variant).mk" "$builddir"/variant.mk
-    # force the reevaluation of this Makefile to make note of the new variant
-    touch ./src/packages/fff/fff/Makefile
+    # apply variant to ensure the included file "variant.mk"
+    # exists in builddir.
+    apply_variant
 
     #saves ~200MB for each build
     test -d ./src/dl || mkdir ./src/dl
@@ -143,6 +142,13 @@ prepare() {
     update_feeds
 }
 
+apply_variant() {
+    # set the variant for this build
+    cp "./src/packages/fff/fff/variant-$(cat selected_variant).mk" "$builddir"/variant.mk
+    # force the reevaluation of this Makefile to make note of the new variant
+    touch ./src/packages/fff/fff/Makefile
+}
+
 update_feeds() {
     ## generate own feeds.conf
     #this local variable should be globally configure variable used in get_source and here
@@ -177,6 +183,8 @@ update_feeds() {
 }
 
 prebuild() {
+    apply_variant
+
     #create filesdir for our config
     /bin/rm -rf "$builddir"/files
     mkdir "$builddir"/files