configure: Don't enable the virtual PNOR by default

To date, most systems are not configured with the virtual PNOR feature.
Turn it off by default, so only systems that want it enable it.

See vpnor/README.md to understand whether this is a feature you want for
your system design.

Change-Id: I33f57946444db92a72851c9ce24df6121860b742
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/configure.ac b/configure.ac
index 5ab386f..d512c01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -58,14 +58,14 @@
 AC_ARG_ENABLE([virtual-pnor],
     AS_HELP_STRING([--enable-virtual-pnor], [Turn on virtual pnor])
 )
-AS_IF([test "x$enable_virtual_pnor" == "xno"],
-    [
-        AM_CONDITIONAL(VIRTUAL_PNOR_ENABLED, false)
-    ],
+AS_IF([test "x$enable_virtual_pnor" == "xyes"],
     [
         AM_CONDITIONAL(VIRTUAL_PNOR_ENABLED, true)
         AX_APPEND_COMPILE_FLAGS([-DVIRTUAL_PNOR_ENABLED], [CXXFLAGS])
         AX_APPEND_COMPILE_FLAGS([-DVIRTUAL_PNOR_ENABLED], [CFLAGS])
+    ],
+    [
+        AM_CONDITIONAL(VIRTUAL_PNOR_ENABLED, false)
     ]
 )