MFC r271097, r271100, r271101, r271102, r271124:

 - Add a basic iomux driver for imx6.
 - Implement the same public interface in imx51 and imx6 iomux
 - The iomux driver is no longer optional, remove it from kernel configs.
 - Implement the imx_iomux_get/set_gpr() interface for imx6.
 - Stop setting the iomux device status to disabled, now that we have a driver.
MFC r266878, r266879: Add support for snprintf() to libstand.
MFC r271285:

  Add a 'ubenv import' command to import environment variables from the
  u-boot env into the loader(8) env (which also gets them into the kernel
  env).