build: use allowed over enabled

Meson feature options are typically in a tri-state of enabled, disabled,
or auto.  The enabled and disabled functions on an option (from
`get_option`) no longer return true for auto features.  Instead, the
expectation is to use `allowed()` which is true for both enabled and auto.

Switch all uses of `enabled` to `allowed`.

Change-Id: I794c658531483266bfce6f9b28127a166ed01490
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/softoff/meson.build b/softoff/meson.build
index 9c8f443..2d672af 100644
--- a/softoff/meson.build
+++ b/softoff/meson.build
@@ -16,7 +16,7 @@
            install: true,
            install_dir: get_option('bindir'))
 
-if get_option('systemd').enabled()
+if get_option('systemd').allowed()
   filesystem.copyfile(
     'services/pldmSoftPowerOff.service',
     'pldmSoftPowerOff.service',