fbt: initial blackmagic support (#1362)

* fbt: added separate script for Windows env setup; moved flash targets from firmware.scons to SConstruct; added Blackmagic support with automatic probe port resolution; added apps.c rebuild on any manifest.fam changes; fixed simultaneous flash & debug ops
* fbt: added networked BlackmagicResolver mode; added `get_blackmagic` target for IDE integration
* fbt: cleanup
* fbt: docs update; fixed blackmagic lookup on certain usb hubs
* fbt: removed explicit python serial port import
* fbt: cleanup
* fbt: raising exception on multiple serial blackmagic probes

site_scons/commandline.scons

@@ -162,6 +162,12 @@ vars.Add(
     default="",
 )
 
+vars.Add(
+    "BLACKMAGIC",
+    help="Blackmagic probe location",
+    default="auto",
+)
+
 vars.Add(
     "UPDATE_SPLASH",
     help="Directory name with slideshow frames to render after installing update package",