fff: create proper package variants instead of copying file

So far, we ensure the selection of a specific variant by copying
included Make files. This not only breaks if the packages are used
as a feed, but also is against the concept of how packages are used.

In this patch, the fff package is converted into a set of variants
that allow selection via a FFF_VARIANT variable that is exported by
buildscript. If no export happens, e.g. when using packages in a
feed, no package is selected.

Since the names fff-node and fff-layer3 are not available anymore,
the packages for the variants are called (though irrelevant for
the user):

  * fff-variant-layer3
  * fff-variant-node

The only drawback is that we now have to specify the list of
available variants in the buildscript. However, these values are
hardcoded in several other places as well, and the former code
based on file names was not really appealing anyway.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

@@ -24,2 +11,2 @@
-			 +fff-timeserver \
-			 +fff-simple-tc
+	URL:=https://www.freifunk-franken.de
+	DEFAULT:=$(if $(findstring fff-variant-$(FFF_VARIANT),$(1)),y,n)

@@ -449,3 +450,3 @@
             echo
         else
-            if [ ! -f "src/packages/fff/fff/variant-$2.mk" ]; then
+            if ! echo "$FFF_VARIANTS" | grep -q "\b$2\b"; then