utils: add trim_version() function

utils: add trim_version() function

Add a helper function that returns just the first <num_parts> of <version>,
split by periods.  For example, trim_version("1.2.3", 2) will return "1.2".

This should help reduce the spread of numerous copies of this idea across
classes and recipes.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>