filter out whitespace from PRODUCT_3RD_PARTY_PACKAGES

The default bdk config will append the variable and inject whitespace
into it.  This isn't a problem normally except that the check to see
if any packages are configured doesn't filter out whitespace.

Change-Id: I6fbbc16cc7c07f4e1b7e44c8e20738ad8d184084
diff --git a/ b/
index e10cad1..ee5778c 100644
--- a/
+++ b/
@@ -89,7 +89,7 @@
 # cases like `make PRODUCT-<product>-eng` which doesn't source
 	$(hide)mkdir -p $(dir $@) $(TARGET_OUT_GENTOO)/tmp
+ifneq ($(strip $(PRODUCT_3RD_PARTY_PACKAGES)),)
 		printf '%s\n' $(3RD_PARTY_PACKAGES_QUOTED) | LC_ALL=C sort > $@.tmp; \
 		if cmp $@.tmp $@ 2>/dev/null; then \