meson: use non-deprecated systemd packageconfig

Systemd's packageconfig file has both `systemdsystemunitdir` and
`systemd_system_unit_dir` defined.  The non-underscore one appears
to be a deprecated alias[1].  Move to the non-deprecated /
underscore-separated variant.

[1]: https://github.com/systemd/systemd/commit/4908de44b0a0409f84a7cdc5641b114d6ce8ba03

Change-Id: Ice33455fe91a14d4235cc4d6c5ea2f4220751ecb
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/firstboot/meson.build b/firstboot/meson.build
index 743eab5..f5156a0 100644
--- a/firstboot/meson.build
+++ b/firstboot/meson.build
@@ -15,7 +15,7 @@
         input: 'first-boot-set-mac@.service',
         output: 'first-boot-set-mac@.service',
         copy: true,
-        install_dir: set_mac.get_variable('systemdsystemunitdir'),
+        install_dir: set_mac.get_variable('systemd_system_unit_dir'),
     )
 endif
 
@@ -30,6 +30,6 @@
         input: 'first-boot-set-hostname.service',
         output: 'first-boot-set-hostname.service',
         copy: true,
-        install_dir: set_hostname.get_variable('systemdsystemunitdir'),
+        install_dir: set_hostname.get_variable('systemd_system_unit_dir'),
     )
 endif
diff --git a/http-redirect/meson.build b/http-redirect/meson.build
index 1d2e612..62b4268 100644
--- a/http-redirect/meson.build
+++ b/http-redirect/meson.build
@@ -15,7 +15,7 @@
             input: unit,
             output: unit,
             copy: true,
-            install_dir: redirect.get_variable('systemdsystemunitdir'),
+            install_dir: redirect.get_variable('systemd_system_unit_dir'),
         )
     endforeach
 endif