Remove global build-std flags

* Use those flags only in specific commands,
  otherwise building some tools from this directory
  will also fail (cargo picks these flags as global).

.cargo/config.toml

@@ -8,6 +8,3 @@ rustflags = [
   "-C", "target-cpu=cortex-a53",
   "-C", "embed-bitcode=yes",
 ]
-
-[unstable]
-build-std = ["core", "compiler_builtins", "alloc"]