bladeRF-cli: Made output formatting more consistent

The preferred output format for commands is a leading newline, content
indented two spaces, and a trailing newline. Sub-levels should be
indented an additional two spaces.

Example:

bladeRF> my_cmd arg1 arg2

  Info1: Something about arg1
  Info2: Something about arg2
    Subinfo 1: abc
    Subinfo 2: xyz

bladeRF> my_cmd badarg arg2

  Error: Message about badarg

bladeRF>
bladeRF-cli: Matched mimo cmd usage to help text, status code cleanup

The help text reports the usage of the mimo command to be:
    mimo [master | slave]

Since there was no alternative to argv[1] being anything other than "clk"
this was dropped so the invocation matches the help text.

If an error occurred, this command was returning a libbladeRF status
code directly, instead of returning a CLI_RET code. This has been fixed.
bladeRF-cli: Added brief summary of how to add new commands
bladeRF-cli: Remove duplicate message from probe when no device is found