devtool: clarify help text

devtool: clarify help text

* Make some minor clarifications to help text
* Drop ArgumentDefaultsHelpFormatter and just put the defaults in the
  text itself where needed (because otherwise you get defaults shown for
  store_true options which is somewhat confusing).

(From OE-Core rev: a90ffea30c4578fd6acda2c5945b816ad33b13f5)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>