Fix hex_from_elf macro for newer bazel versions

The `$(OBJCOPY)` make variable is only available if you explicitly
depend on the C++ toolchain.

Change-Id: Ic17dd6e858d11753b0b319818ccc7b3d40c9ba02
diff --git a/motors/macros.bzl b/motors/macros.bzl
index 4f3cf17..6b15f00 100644
--- a/motors/macros.bzl
+++ b/motors/macros.bzl
@@ -7,4 +7,5 @@
         executable = True,
         output_to_bindir = True,
         restricted_to = restricted_to,
+        toolchains = ["@bazel_tools//tools/cpp:current_cc_toolchain"],
     )