phosphor-image-updater: move to libexec

Non-user executables are suppose to be installed into libexec[1].

[1]: https://github.com/openbmc/docs/blob/master/anti-patterns.md#incorrect-placement-of-executables-in-sbin-usrsbin-or-bin-usrbin

Tested:

```
$ meson install -C builddir --destdir /tmp/phosphor-bmc-code-mgmt
$ find /tmp/phosphor-bmc-code-mgmt/ -name phosphor-image-updater
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-image-updater
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iba20534db7404bf341d33bc2c5d8d885cade8fff
diff --git a/bmc/meson.build b/bmc/meson.build
index af7cc74..7909c58 100644
--- a/bmc/meson.build
+++ b/bmc/meson.build
@@ -218,6 +218,7 @@
     'item_updater_main.cpp',
     dependencies: [deps, ssl, boost_dep],
     install: true,
+    install_dir: get_option('libexecdir') / 'phosphor-code-mgmt',
 )
 
 executable(
diff --git a/bmc/xyz.openbmc_project.Software.BMC.Updater.service.in b/bmc/xyz.openbmc_project.Software.BMC.Updater.service.in
index 36be143..bb05629 100644
--- a/bmc/xyz.openbmc_project.Software.BMC.Updater.service.in
+++ b/bmc/xyz.openbmc_project.Software.BMC.Updater.service.in
@@ -4,7 +4,7 @@
 Before=xyz.openbmc_project.Software.Version.service
 
 [Service]
-ExecStart=/usr/bin/phosphor-image-updater
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-image-updater
 Restart=always
 Type=dbus
 BusName={BUSNAME}