tcl/board: Move SiFive configs into vendor directory

Move the configuration files into a dedicated vendor folder as required
by the developer guidelines.

Change-Id: I5bf048f2d8d0fccbcfe40e0a0e7b30dfbab192d1
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/9082
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

src/helper/startup.tcl

@@ -39,7 +39,7 @@ proc find {filename} {
 		return $t
 	}
 
-	foreach vendor {nordic ti st} {
+	foreach vendor {nordic ti sifive st} {
 		# - path/to/a/certain/config_file
 		# replaced with
 		# - path/to/a/certain/${vendor}/config_file