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: Icc698b95d762197d496ec9f228a12d7fe48beef0
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/test/meson.build b/test/meson.build
index 822a4f8..e5ab932 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -19,7 +19,7 @@
   endif
 endif
 
-if not get_option('openpower-pel-extension').disabled()
+if get_option('openpower-pel-extension').allowed()
     subdir('openpower-pels')
 endif