firmware: add missing usb strings to blupdate that prevented building it

Change-Id: I8d0d9bf98748d475427d9b0c78f2e0532ee4bca1
diff --git a/firmware/apps/blupdate/usb_strings.txt b/firmware/apps/blupdate/usb_strings.txt
new file mode 100644
index 0000000..f652071
--- /dev/null
+++ b/firmware/apps/blupdate/usb_strings.txt
@@ -0,0 +1,6 @@
+sysmocom - s.f.m.c. GmbH
+PRODUCT_STRING
+bootloader updater
+RAM
+Flash (Application Partition)
+Flash (Bootloader Partition)