tools/utils: Add function to filter files from the project

A new function is introduced called `get_filtered_files` to
recursively find and filter files in the current working directory
and its subdirectories. It allows to simplify the check scripts.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I10707e4f2cbc53b32194514a543e2d244e18356c