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/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 \