phosphor-usb-code-update: 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-usb-code-update
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-usb-code-update
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I59925038785709c3bfb2e374c4a233e2cd9d9176
diff --git a/bmc/usb/meson.build b/bmc/usb/meson.build
index b90f370..c531fab 100644
--- a/bmc/usb/meson.build
+++ b/bmc/usb/meson.build
@@ -20,7 +20,7 @@
         sdeventplus_dep,
     ],
     install: true,
-    install_dir: get_option('bindir'),
+    install_dir: get_option('libexecdir') / 'phosphor-code-mgmt',
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
diff --git a/bmc/usb/services/usb-code-update@.service b/bmc/usb/services/usb-code-update@.service
index ebb3ac1..16d268c 100644
--- a/bmc/usb/services/usb-code-update@.service
+++ b/bmc/usb/services/usb-code-update@.service
@@ -8,6 +8,6 @@
 
 [Service]
 Restart=no
-ExecStart=/usr/bin/phosphor-usb-code-update --device %i
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-usb-code-update --device %i
 Type=simple
 RemainAfterExit=yes