artiq_flash: cleanup openocd handling, do not follow symlinks

Not following symlinks allows files to be added to OpenOCD via nixpkgs buildEnv.

artiq/frontend/artiq_flash.py

@@ -75,28 +75,22 @@ Prerequisites:
                         help="actions to perform, default: %(default)s")
     return parser
 
-def which_openocd():
+def openocd_root():
     openocd = shutil.which("openocd")
     if not openocd:
         raise FileNotFoundError("OpenOCD is required but was not found in PATH. Is it installed?")
-    return openocd
+    return os.path.dirname(os.path.dirname(openocd))
+
 
 def scripts_path():
     p = ["share", "openocd", "scripts"]
     if os.name == "nt":
         p.insert(0, "Library")
-    p = os.path.abspath(os.path.join(
-        os.path.dirname(os.path.realpath(which_openocd())),
-        "..", *p))
-    return p
+    return os.path.abspath(os.path.join(openocd_root(), *p))
 
 
 def proxy_path():
-    p = ["share", "bscan-spi-bitstreams"]
-    p = os.path.abspath(os.path.join(
-        os.path.dirname(os.path.realpath(which_openocd())),
-        "..", *p))
-    return p
+    return os.path.abspath(os.path.join(openocd_root(), "share", "bscan-spi-bitstreams"))
 
 
 def find_proxy_bitfile(filename):