config_system: return a set from dependency_list
config_system: return a set from dependency_list

This avoids adding duplicate dependencies unneccesarily.

Change-Id: I172d9661545f2c1a70a4d471fc8fe187c91832d0
Signed-off-by: David Kilroy <159cd46f289d7961b38b0171b53c05be4aaef0b6@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build