In structure XML files, implement component library files inclusion

BZ: 168727

In the PFW structure file, it is not possible to include a component
library from another XML file.

Implement the possibility to import component from another XML file
that would be included in a structure XML file and that would
describe a component library.

Change-Id: Id6125140de1c8e9882375d01199f695b929f45e2
Signed-off-by: Guillaume Denneulin <guillaume.denneulin@intel.com>