Better handling of duplicate default

Currently, an error is raised if more than one default is defined.

When including another manifest, it is likely that a default has
been defined in both manifests.

Don't raise an error if all the defaults defined have the same
attributes.

Change-Id: I2603020687e2ba04c2c62c3268ee375279b34a08
Signed-off-by: Julien Campergue <julien.campergue@parrot.com>