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: I4db060ea85cfae7be96693ec04a09829767351a1
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/fault-monitor/meson.build b/fault-monitor/meson.build
index 39ec458..c314665 100644
--- a/fault-monitor/meson.build
+++ b/fault-monitor/meson.build
@@ -2,7 +2,7 @@
     'monitor-main.cpp',
 ]
 
-if get_option('monitor-operational-status').enabled()
+if get_option('monitor-operational-status').allowed()
     fault_monitor_sources += [
         '../utils.cpp',
         'operational-status-monitor.cpp',