openpower-pnor: don't commit type crimes

Currently, the BR2_OPENPOWER_PNOR_XZ_ENABLED variable is a string, that
can contain the values "true" or "false".

This change uses a proper boolean type, and removes the double negative
in the comment.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/openpower/package/openpower-pnor/Config.in b/openpower/package/openpower-pnor/Config.in
index 6d4193b..a96f042 100644
--- a/openpower/package/openpower-pnor/Config.in
+++ b/openpower/package/openpower-pnor/Config.in
@@ -84,6 +84,5 @@
             String used to define name of openpower targeting binary file, ecc protected
 
 config BR2_OPENPOWER_PNOR_XZ_ENABLED
-        string "False if we are not compressing with XZ anywhere"
-        default "false" if !BR2_TARGET_SKIBOOT_XZ
-        default "true" if BR2_TARGET_SKIBOOT_XZ
+        bool "Enable xz compression in PNOR payloads"
+        default y if BR2_TARGET_SKIBOOT_XZ