base-hw: preserve symbolic link bin/core

In this case "mv A B" works slightly different than "cp A B; rm A" as
symbolic links come into play. The statements should copy the contents
of A into the symboliv link at B (preserving it as is) and remove A. The
mv would replace the link B by the binary A.

Fixes #805.

base-hw/run/env

@@ -207,7 +207,8 @@ proc build_boot_image {binaries} {
 	}
 
 	# retrieve stand-alone core
-	exec mv core/core.standalone bin/core
+	exec cp core/core.standalone bin/core
+	exec rm core/core.standalone
 }