overlay: Don't mount /boot/efi by default

Nothing in the OS touches the ESP by default, so there's
no reason to mount it by default, particularly writable.
This is good for avoiding wear&tear on the filesystem, but
I am specifically doing this as preparation for potentially
removing the ESP from AWS images, because AWS `ImportImage`
chokes on its presence:
openshift/os#396

overlay.d/05core/usr/lib/systemd/system-generators/coreos-boot-mount-generator

@@ -45,9 +45,12 @@ EOF
     # which are 1) machines actually booted through EFI, and 2) x86_64
     # when booted through BIOS.
     if [ "$(uname -m)" = "x86_64" -o -d /sys/firmware/efi ]; then
-        add_wants boot-efi.mount
         cat > "${UNIT_DIR}/boot-efi.mount" <<EOF
-# Automatically created by coreos-boot-mount-generator
+# Automatically created by coreos-boot-mount-generator; not
+# mounted by default though, since updates don't touch it.
+# See also https://github.com/ostreedev/ostree/pull/1873
+# And in the future we might omit this partition at least
+# AWS because their `ImportImage` API call chokes on it.
 [Unit]
 Description=EFI System Partition
 Documentation=https://github.com/coreos/fedora-coreos-config