USB: persistent naming for USB-Serial converters

In case a BPM is connected to an USB port via a converter, the USBPort property of the device server has to be set. This can be done in three different ways:

USBPort can be set to the serial number identifying the converter, e.g.: FTE30LRC
This is the recommended procedure. It is safe as long as a BPM is connected to the same converter. The Tango server identifies the correct USB port by fetching the serial no. of the converter connected to the port.
A serial number is determined by, e.g.:
```
 
udevadm info -a -n /dev/ttyUSB0 | grep {serial} | head -n 1
```
USBPort can be set to a symbolic link pointing to the device file, e.g.: /dev/BPM4.
This method is also safe since it also depends on the serial no. of the converter. However, it requires that udev rules have been defined that create the sym-links.
USBPort can be set to the device file, e.g.: /dev/ttyUSB0.
Warning: the order of the USB devices is not predictable. After a converter has been connected, the command dmesg tells you which device has been created. If the BPM has been repowered or reconnected the USB port has to be checked again.

This is the current configuration of the BPMs which are connected via USB:

haspp03: I404/EXP,       p03/i404/exp.01,   USBPort: ['/dev/ttyUSB0'] FTE30LRC

haspp03bpmhost: I404/OH, p03/i404/mono.01,  USBPort: ['/dev/ttyUSB0'] FTXYEBXN

haspp06mc01: I404/MICRO, p06/i404/micro.01, USBPort: ['/dev/ttyUSB0'] FTUJ0VI3

haspp06mono: I404/MONO,  p06/i404/mono.01,  USBPort: ['/dev/ttyUSB0'] FTU7MSFU

haspp08ohbpm: I404/BPM1, p08/i404/exp.01,   USBPort: ['/dev/ttyUSB0'] FTYRYR61
haspp08ohbpm: I404/BPM2, p08/i404/exp.02,   USBPort: ['/dev/ttyUSB1'] FTYRYSZW
haspp08ohbpm: I404/BPM3, p08/i404/exp.03,   USBPort: ['/dev/ttyUSB2'] FTYRTDLD

haspp08gpib: I404/BPM4,  p08/i404/exp.04,   USBPort: ['/dev/ttyUSB1'] FTYRYSSJ

haspp09: I404/BPM-EH1,   p09/i404/exp.01,   USBPort: ['/dev/ttyUSB0'] FTUJ3YG2

haspp10e1: I404/P10E1,   p10/i404/e1.01,    USBPort: ['/dev/ttyUSB0'] FTXYEJXJ