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: I0c1bf5bea775562681d09e61ad7358eeb4de5c80
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 40bcf38..7ccc92c 100644
--- a/meson.build
+++ b/meson.build
@@ -191,7 +191,7 @@
     include_type: 'system',
 )
 
-if get_option('oem-ampere').enabled()
+if get_option('oem-ampere').allowed()
     add_project_arguments('-DOEM_AMPERE', language : 'cpp')
 endif
 
@@ -229,7 +229,7 @@
 ]
 
 oem_files = []
-if get_option('oem-ampere').enabled()
+if get_option('oem-ampere').allowed()
     subdir('oem/ampere')
 endif