coremanager: allow sub-directories in cores/systems

With this new features, we can have multiples sub-directories
in systems and cores directories. For example:

|
|-- de1
|   |-- de1
|   |-- de1-blink
|   `-- de1-eco32

This allows systems to be grouped under board name.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>