digilent: add driver skeleton + CMake option BS_ENABLE_DIGILENT

Stub Digilent JTAG-SMT backend, off by default. Wiring only: option,
conditional sources, dl link, drivers_list registration. Detect()
returns 0 for now; dlopen + real implementation in follow-up commits.

modules/drivers/drivers_list.c

@@ -40,6 +40,10 @@
 #include "./jlink_jtag/jlink_jtag_drv.h"
 #endif
 
+#ifdef BS_ENABLE_DIGILENT
+#include "./digilent_jtag/digilent_jtag_drv.h"
+#endif
+
 #include "drivers_list.h"
 
 const drv_entry staticdrvs[] =
@@ -57,6 +61,9 @@ const drv_entry staticdrvs[] =
 #endif
 #if defined(__linux__)
 	{(DRV_GETMODULEINFOS)drv_LinuxGPIO_libGetDrv,0},
+#endif
+#ifdef BS_ENABLE_DIGILENT
+	{(DRV_GETMODULEINFOS)drv_Digilent_libGetDrv,0},
 #endif
 	{(DRV_GETMODULEINFOS)-1,0}
 };