atomic: add Developer Mode boot menu item

This patch is for the spin-kickstarts repository.

Call out to the `bootentry` script in the kickstart %post so that the
Developer Mode option is added to the GRUB 2 menu.

fedora-cloud-atomic.ks

@@ -122,5 +122,8 @@ dd bs=1M if=/dev/zero of=/var/tmp/zeros || :
 rm -f /var/tmp/zeros
 echo "(Don't worry -- that out-of-space error was expected.)"
 
+echo "Adding Developer Mode GRUB2 menu item."
+/usr/libexec/atomic-devmode/bootentry add
+
 %end