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
diff --git a/openpower/package/openpower-pnor/openpower-pnor.mk b/openpower/package/openpower-pnor/openpower-pnor.mk
index 065cd85..b314538 100644
--- a/openpower/package/openpower-pnor/openpower-pnor.mk
+++ b/openpower/package/openpower-pnor/openpower-pnor.mk
@@ -58,7 +58,7 @@
             -capp_binary_filename $(BINARIES_DIR)/$(BR2_CAPP_UCODE_BIN_FILENAME) \
             -openpower_version_filename $(OPENPOWER_PNOR_VERSION_FILE) \
             -payload $(BINARIES_DIR)/$(BR2_SKIBOOT_LID_NAME) \
-            -xz_compression $(BR2_OPENPOWER_PNOR_XZ_ENABLED)
+            $(if ($(BR2_OPENPOWER_PNOR_XZ_ENABLED),y),-xz_compression true)
 
         mkdir -p $(STAGING_DIR)/pnor/
         $(TARGET_MAKE_ENV) $(@D)/create_pnor_image.pl \