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: I816b3334e4bb65384a49bc34af6ae5fe092c923e
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 943cd37..c8f0c31 100644
--- a/meson.build
+++ b/meson.build
@@ -22,7 +22,7 @@
     configuration: conf_data
 )
 
-if get_option('oe-sdk').enabled()
+if get_option('oe-sdk').allowed()
     OECORE_TARGET_SYSROOT = run_command('sh', '-c', 'echo $OECORE_TARGET_SYSROOT').stdout().strip()
     if OECORE_TARGET_SYSROOT == ''
         error('OECORE_TARGET_SYSROOT must be set with enable oe-sdk')