build: use allowed over enabled or not-disabled

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`.
Switch all uses of `not disabled` to `allowed`.

Change-Id: If5d6047137a085eaa6a3de0580aff96488596825
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/vpd-manager/meson.build b/vpd-manager/meson.build
index 4e9c10b..3a3710e 100644
--- a/vpd-manager/meson.build
+++ b/vpd-manager/meson.build
@@ -18,7 +18,7 @@
 parser_dependencies = [sdbusplus, libgpiodcxx, phosphor_logging, phosphor_dbus_interfaces]
 
 parser_build_arguments = []
-if get_option('ibm_system').enabled()
+if get_option('ibm_system').allowed()
     parser_build_arguments += ['-DIBM_SYSTEM']
 endif