MLBEDSW-4783: Make config handling more user friendly

Vela was not able to parse config file paths entered with forward
slashes. This patch will make it possible to use both forward and
backslashes when specifying paths.

Signed-off-by: Rickard Bolin <rickard.bolin@arm.com>
Change-Id: I0f4cfc16bde5738c73059af6216d2bdc3821c68b