[i.MX6][i.MX8][ENV] Add autocompletion to seco_config

This adds autocompletion with TAB to seco_config commands.

MX SECO U-Boot > seco_config f
  fdtsrc fssrc fwreset fdtoverlay

Also minor adjustments to the help section, as some options were missing or not supported.

Tested on imx8 version, but imx6 one is quite similar.