boot-time-monitor: store boot time data in /var/google

All data in /var/google persists after update. Since boot time monitor
service needs data that stored before powercycle, moving the data
directory to /var/google.

Tested:
```
root@ddcgx3-nfd01:~# ls /usr/share/boot_time_monitor/
ls: /usr/share/boot_time_monitor/: No such file or directory
root@ddcgx3-nfd01:~# ls /var/google/boot_time_monitor/
bmc_checkpoints.csv            bmc_checkpoints.csv.completed  bmc_durations.csv              bmc_durations.csv.completed    host0_checkpoints.csv          host0_durations.csv
```

Google-Bug-Id: 326579702
Change-Id: Ia9cd39ac18311dd3e8aa5813f02dd2040dfb1ece
Signed-off-by: Munawar Hussain <munawarhussain@google.com>
(cherry pick from commit e5fdcc6cac5ee62a83fc1f949f80fc896ef588ac)
diff --git a/recipes-google/boot/boot-time-monitor_git.bb b/recipes-google/boot/boot-time-monitor_git.bb
index a12d267..b2dc812 100644
--- a/recipes-google/boot/boot-time-monitor_git.bb
+++ b/recipes-google/boot/boot-time-monitor_git.bb
@@ -20,6 +20,7 @@
 EXTRA_OEMESON:append = " \
     -Dtests=disabled \
     -Dyocto=enabled \
+    -Ddata-directory='/var/google/boot_time_monitor/' \
 "
 
 SYSTEMD_PACKAGES = "${PN}"
@@ -36,5 +37,5 @@
 "
 
 do_install:append() {
-  install -d ${D}${datadir}/boot_time_monitor
+  install -d ${D}/var/google/boot_time_monitor
 }