[antlir2] package: allow selector for BtrfsSubvol

Summary:
For default VM, the layer target is going to change based on arch when
constructing the disk. Allow selector.

Test Plan: CI. Boot VM with BtrfsSubvol (V0.16)

Reviewed By: sergeyfd

Differential Revision: D47570798

fbshipit-source-id: 9a5d945d8e0a6201d1f8a64679c15d1cb59f5009

antlir/antlir2/bzl/package/defs.bzl

@@ -101,7 +101,7 @@ _package = rule(
 _package_macro = rule_with_default_target_platform(_package)
 
 def BtrfsSubvol(
-        layer: str.type,
+        layer: [str.type, "selector"],
         writable: [bool.type, None] = None):
     return {
         "layer": layer,