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: If4484727bbd187b6fb92b7a0997fb8fcda5aec70
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 0cfd2b4..f5a1d74 100644
--- a/meson.build
+++ b/meson.build
@@ -33,7 +33,7 @@
         dependency('systemd'),
 ]
 
-if(get_option('usb-code-update').enabled())
+if(get_option('usb-code-update').allowed())
     add_project_arguments('-DUSB_CODE_UPDATE', language : 'cpp')
 endif