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/libboard/octsimtest/product_string.txt b/firmware/libboard/octsimtest/product_string.txt
new file mode 100644
index 0000000..83cbcf5
--- /dev/null
+++ b/firmware/libboard/octsimtest/product_string.txt
@@ -0,0 +1 @@
+sysmoOCTSIM-Tester