Allow to select FTDI device by serial number

Requires libftdi > 0.5 (2004, commit ID a8f46ddc1595b1b07abfcce613acdafe5b8ddf9d).
Idea stolen from chromiumos commit 5eb5624aeb7e2ee483e2fa0823c4e634c8ea3e68:
http://git.chromium.org/gitweb/?p=chromiumos/third_party/flashrom.git;a=commit;h=5eb5624aeb7e2ee483e2fa0823c4e634c8ea3e68

Corresponding to flashrom svn r1599.

Signed-off-by: Shik Chen <shik@chromium.org>
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
diff --git a/flashrom.8 b/flashrom.8
index bf671c2..cf07ccf 100644
--- a/flashrom.8
+++ b/flashrom.8
@@ -573,6 +573,15 @@
 and the default interface is
 .BR B .
 .sp
+If there is more than one ft2232_spi-compatible device connected, you can select which one should be used by
+specifying its serial number with the
+.sp
+.B "  flashrom \-p ft2232_spi:serial=number"
+.sp
+syntax where
+.B number
+is the serial number of the device (which can be found for example in the output of lsusb -v).
+.sp
 All models supported by the ft2232_spi driver can configure the SPI clock rate by setting a divisor. The
 expressible divisors are all even numbers between 2 and 2^17 (=131072) resulting in SPI clock frequencies of
 6 MHz down to about 92 Hz for 12 MHz inputs. The default divisor is set to 2, but you can use another one by