overlay/coreos-boot-mount-generator: Small code simplification

Prep for further work.  systemd requires an exact relationship
between the unit name and mount point, so rather than having
the caller pass both, compute the unit name from the mount.

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

@@ -33,11 +33,12 @@ fi
 # device that systemd will fsck. This code ensures that if the label
 # is backed by a device-mapper target the dev-mapper.*.device is used.
 mk_mount() {
-    local unit_name="${1}.mount"; shift
+    local mount_pt="${1}"; shift
     local label="${1}"; shift
-    local mount_pt="${1:-/$label}"
     local path="/dev/disk/by-label/${label}"
 
+    local unit_name=$(systemd-escape -p ${mount_pt} --suffix=mount)
+
     eval $(udevadm info --query property --export "${path}")
     device="$(systemd-escape ${path})"
     if [ "${DM_NAME:-x}" != "x" ]; then
@@ -70,13 +71,13 @@ EOF
 # ConditionPathExists won't work here because conditions don't affect
 # the dependency on the underlying device unit.
 if [ ! -f /run/ostree-live ]; then
-    mk_mount boot boot
+    mk_mount /boot boot
 
     # Only mount the EFI System Partition on machines where it exists,
     # 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
-        mk_mount boot-efi EFI-SYSTEM "/boot/efi"
+        mk_mount /boot/efi EFI-SYSTEM
         # In the general case the ESP might have per-machine or private
         # data on it.  Let's not make it world readable on general
         # principle.