extlinux-conf-builder: Choose whether to use DTBs on per-generation basis

Previously, the value from stdenv.platform.kernelDTB was used. That
doesn't work well if both kinds (DTB and non-DTB) of generations exist
in the system profile.

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.nix

···
   isExecutable = true;
   path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
   inherit (pkgs) bash;
-  kernelDTB = pkgs.stdenv.platform.kernelDTB or false;
 }

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.sh

···
 
     copyToKernelsDir "$path/kernel"; kernel=$result
     copyToKernelsDir "$path/initrd"; initrd=$result
-    if [ -n "@kernelDTB@" ]; then
-        # XXX UGLY: maybe the system config should have a top-level "dtbs" entry?
-        copyToKernelsDir $(readlink -m "$path/kernel/../dtbs"); dtbs=$result
+    # XXX UGLY: maybe the system config should have a top-level "dtbs" entry?
+    dtbDir=$(readlink -m "$path/kernel/../dtbs")
+    if [ -d "$dtbDir" ]; then
+        copyToKernelsDir "$dtbDir"; dtbs=$result
     fi
 
     timestampEpoch=$(stat -L -c '%Z' $path)
···
     fi
     echo "  LINUX ../nixos/$(basename $kernel)"
     echo "  INITRD ../nixos/$(basename $initrd)"
-    if [ -n "@kernelDTB@" ]; then
+    if [ -d "$dtbDir" ]; then
         echo "  FDTDIR ../nixos/$(basename $dtbs)"
     fi
     echo "  APPEND systemConfig=$path init=$path/init $extraParams"