Swap to using the USERSPACE_ARCH instead of QEMU's

Debian uses a different set of architecture names, and QEMU's names do
not line up with the archive's (ie: aarch64 vs. arm64, armhf vs. arm).

Change-Id: I8389ce2ac7fbb39872a4c6a1e358d4343d119df8
diff --git a/packages.mk b/packages.mk
index d5cbe24..167a281 100644
--- a/packages.mk
+++ b/packages.mk
@@ -45,7 +45,7 @@
 		--basetgz $@ \
 		--othermirror "deb [trusted=yes] http://packages.cloud.google.com/apt mendel-day main|deb [trusted=yes] http://packages.cloud.google.com/apt mendel-bsp-$(BOARD_NAME)-day main" \
 		--distribution buster \
-		--architecture $(QEMU_ARCH) \
+		--architecture $(USERSPACE_ARCH) \
 		--extrapackages "build-essential debhelper gnupg lintian" \
 		--aptcache ""
 	mkdir -p $(ROOTDIR)/cache/base-tmp