jtag/adapter: add command 'adapter serial'

Several adapter define their own command to specify the USB serial
number or serial string to be used during USB search.

Define a general command 'adapter serial' to be proposed as
replacement of the driver specific ones.
No driver is changed so far to use it.

Change-Id: I7631687a4163ccc63a9bdf3ad1fcb300fc483d3a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6647
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins

src/jtag/adapter.h

@@ -55,4 +55,7 @@ int adapter_config_rclk(unsigned int fallback_speed_khz);
 /** Retrieves the clock speed of the adapter in kHz. */
 unsigned int adapter_get_speed_khz(void);
 
+/** Retrieves the serial number set with command 'adapter serial' */
+const char *adapter_get_required_serial(void);
+
 #endif /* OPENOCD_JTAG_ADAPTER_H */