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: I0ae94b17ccfe82c1fbc3e4e0602b0a6ecd21762d
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/bios/meson.build b/bios/meson.build
index 8e0092a..5515e2a 100644
--- a/bios/meson.build
+++ b/bios/meson.build
@@ -23,7 +23,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )
 
diff --git a/bmc/side-switch/meson.build b/bmc/side-switch/meson.build
index e04a456..9e44852 100644
--- a/bmc/side-switch/meson.build
+++ b/bmc/side-switch/meson.build
@@ -10,7 +10,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )
 
diff --git a/bmc/usb/meson.build b/bmc/usb/meson.build
index c531fab..386c943 100644
--- a/bmc/usb/meson.build
+++ b/bmc/usb/meson.build
@@ -24,7 +24,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )
 udev_dir = dependency('udev').get_variable(
diff --git a/cpld/meson.build b/cpld/meson.build
index 2ab8793..cb5c706 100644
--- a/cpld/meson.build
+++ b/cpld/meson.build
@@ -21,7 +21,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )
 
diff --git a/eeprom-device/meson.build b/eeprom-device/meson.build
index 2e8edd7..7dafd7b 100644
--- a/eeprom-device/meson.build
+++ b/eeprom-device/meson.build
@@ -24,7 +24,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )
 
diff --git a/i2c-vr/meson.build b/i2c-vr/meson.build
index 2af9c56..13e1b37 100644
--- a/i2c-vr/meson.build
+++ b/i2c-vr/meson.build
@@ -25,7 +25,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )
 
diff --git a/meson.build b/meson.build
index f1a5428..0bc6b7c 100644
--- a/meson.build
+++ b/meson.build
@@ -60,7 +60,7 @@
 ssl = dependency('openssl')
 
 systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_variable('systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable('systemd_system_unit_dir')
 
 build_tests = get_option('tests')