meson: remove deprecated get_pkgconfig_variable

Since meson 0.56, the `get_pkgconfig_variable` has been deprecated.  In
meson 0.58 the `get_variable` was enhanced to no longer require the
`pkgconfig` keyword argument.  Ensure meson 0.58 is required and update
the usage of all `get_pkgconfig_variable` and `get_variable` to be the
modern variant.

Change-Id: I46c094c1a1e4793556e93f9e5158bff7c6042cc4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 0d9d68d..0d1c218 100644
--- a/meson.build
+++ b/meson.build
@@ -5,7 +5,7 @@
         'warning_level=3',
         'werror=true'
     ],
-    meson_version: '>= 0.57.0',
+    meson_version: '>= 0.58.0',
     license: 'Apache-2.0',
     version: '1.0')
 
@@ -140,7 +140,7 @@
 ssl = dependency('openssl')
 
 systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_variable(pkgconfig : 'systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable('systemdsystemunitdir')
 
 unit_files = [
     'obmc-flash-bmc-setenv@.service.in',