build: install three targets to handle bmc updates

Transition from three service files to three targets installed by the
BMC library:
 - phosphor-ipmi-flash-bmc-prepare.target
 - phosphor-ipmi-flash-bmc-verify.target
 - phosphor-ipmi-flash-bmc-update.target

The prepare target is started each time the firmware update state
machine changes from notYetStarted to uploadInProgress and can be
leveraged to free memory (flush caches) or anything else required.

The verify target is started when the verify blob is committed.  The
implementation of services within this target are such that they are
expected to output a status value that's readable (typically via a
file).

The update target is started when the update blob is committed.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I1e838de954a3cf8956ff4b72d090e253b832efe7
diff --git a/bmc/phosphor-ipmi-flash-bmc-prepare.target.in b/bmc/phosphor-ipmi-flash-bmc-prepare.target.in
new file mode 100644
index 0000000..7f0cbd9
--- /dev/null
+++ b/bmc/phosphor-ipmi-flash-bmc-prepare.target.in
@@ -0,0 +1,2 @@
+[Unit]
+Description=Phosphor-ipmi-flash Prepare BMC to receive update