sstorage: sync build variables for flexible cross build

Sync Makefile with the ones provided by the other examples, allowing the
build variables to be set by the build environment, which is specially
useful with OpenEmbedded.

Also sync build flags such as --no-builtin-variables and LDFLAGS.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/secure_storage/Makefile b/secure_storage/Makefile
index 3b9a8dc..dfa4f8b 100644
--- a/secure_storage/Makefile
+++ b/secure_storage/Makefile
@@ -6,8 +6,8 @@
 
 .PHONY: all
 all:
-	$(MAKE) -C host CROSS_COMPILE="$(HOST_CROSS_COMPILE)"
-	$(MAKE) -C ta CROSS_COMPILE="$(TA_CROSS_COMPILE)"
+	$(MAKE) -C host CROSS_COMPILE="$(HOST_CROSS_COMPILE)" --no-builtin-variables
+	$(MAKE) -C ta CROSS_COMPILE="$(TA_CROSS_COMPILE)" LDFLAGS=""
 
 .PHONY: clean
 clean: