MLBEDSW-4783: Add config file directory structure

Add directory structure to support third party config files. Config
files should now be placed in an appropriately named directory under
the config_files directory, but can also be accessed by providing its
absolute path to vela --config.

Signed-off-by: Rickard Bolin <rickard.bolin@arm.com>
Change-Id: I2fcf52e7b2ddd2c4491dc370c85c0b3937d18062