Check if data is available for debstuff

src/include/build.sh

     # read content of each mandatory file from debian_skel
     #
     local debian_skel=$(ini 1value debstuff:debian_skel)
+    test -n "$debian_skel" || die "debstuff:debian_skel not specified"
+    test -d "$debian_skel" || die "debian directory template found: $debian_skel"
     mkdir -p debian/source
     local dfsrc dftgt
     find "$debian_skel" -type f \