openpower-pnor: Rework dependency on enabling XZ

Rather than having all dependencies expressed in
BR2_OPENPOWER_PNOR_XZ_ENABLED, have its dependents do a 'select'
instead.

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 a96f042..7719eda 100644
--- a/openpower/package/openpower-pnor/Config.in
+++ b/openpower/package/openpower-pnor/Config.in
@@ -48,6 +48,7 @@
 
 config BR2_TARGET_SKIBOOT_XZ
         boolean "Compress the skiboot image with XZ"
+        select BR2_OPENPOWER_PNOR_XZ_ENABLED
         default y
 
 config BR2_SKIBOOT_LID_XZ_NAME
@@ -85,4 +86,4 @@
 
 config BR2_OPENPOWER_PNOR_XZ_ENABLED
         bool "Enable xz compression in PNOR payloads"
-        default y if BR2_TARGET_SKIBOOT_XZ
+        default n