Remove -separate-debug-info option for i.MX6

It seems like this option is not supported for this configuration

  Separate debug info support disabled.
  ERROR: -separate-debug-info was requested but this binutils does not support it.

Change-Id: I780ed1f0696d2ea886d4a0905991f59e0e72c58f
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>