Executes 'make dirclean' only if $GLUON_BUILDDIR exists.

By now, 'make dirclean' got executed for branches not equal to 'exper-
imental' no matter if one just cloned all the repos or tries to rebuild
an image. Hence, in the former case where there is nothing to clean,
make complains that 'make dirclean' fails as $GLUON_BUILDDIR does not
exist yet.

Thus, we now test whether or not the directory exists before calling
'make dirclean' and only call it, if it does not exist (i.e. one just
cloned the repos).

Keep in mind that 'make dirclean' still does not get called in case BRANCH is
set to 'experimental'.

Closes issue #20.

build.sh

@@ -92,7 +92,7 @@ SITE_COMMIT=$(git rev-list --max-count=1 HEAD)
 cd ..
 
 ### CLEAN
-if [ "$BRANCH" != "experimental" ]; then
+if [ -d "./build/" -a "$BRANCH" != "experimental" ]; then
     progress "Cleaning your build environment ..."
     make dirclean
 fi