Patch actual board name into the USB iProduct string descriptor

"SIMtrace 2 compatible device" is pretty generic.  Let's have the
actual board name inside the string descriptors, giving a more
user friendly experience in case users are issuing 'lsusb' and the
like.

Change-Id: Ibcc338b504bd2a1605e31d7f5eadb7161f547c6a
diff --git a/firmware/apps/trace/usb_strings.txt b/firmware/apps/trace/usb_strings.txt
index f0cd24c..3a1bcc3 100644
--- a/firmware/apps/trace/usb_strings.txt
+++ b/firmware/apps/trace/usb_strings.txt
@@ -1,5 +1,5 @@
 sysmocom - s.f.m.c. GmbH
-SIMtrace 2 compatible device
+PRODUCT_STRING
 SIMtrace Sniffer
 SIMtrace CCID
 SIMtrace Card Emulation