devtool: show some warnings for upgrade versions

devtool: show some warnings for upgrade versions

* Show a warning in devtool upgrade if the version is less than the
  current version suggesting that the user may need to bump PE in the
  recipe
* Show a warning in devtool add and devtool upgrade if the version looks
  like a pre-release version suggesting using a version number that
  won't mess up the progression when you come to upgrade to the final
  release version.

(From OE-Core rev: 92c4d9be9ed365c449ea1ac5a18ddb15e4a36b2d)

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>