u-boot sys_eeprom: Add/Revise the command help messages

- Show message after executing the 'read' and 'erase' commands.
- Revise the help messages of 'usage', 'read', and 'erase'.
- Revise the 'read' command to always load EEPROM data from device to
  memory.
This message end with a double new line "\n\n".  Please do not do that.  No other output in the sys_eeprom command uses "\n\n".  Let's keep it consistent.