Generate USB Strings from apps/*/usb_strings.txt files at compile time

This way we can skip the manually-crafted USB string definitions in the
dfu_desc.c and usb.c files.
diff --git a/firmware/apps/dfu/usb_strings.txt b/firmware/apps/dfu/usb_strings.txt
new file mode 100644
index 0000000..bdf2048
--- /dev/null
+++ b/firmware/apps/dfu/usb_strings.txt
@@ -0,0 +1,5 @@
+sysmocom - s.f.m.c. GmbH
+SIMtrace 2 compatible device
+DFU (Device Firmare Upgrade)
+RAM
+Flash (Application Partition)