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: I0c79d23d21956ffd007b1d1001bf6aaa0f72103b
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>