Add API to create fru records only for present frus.

This commit builds the fru records only for the present
frus. Any item which is a FRU should by default implement
the Inventory.Item Interface and host the present property.
So, if the present property is not hosted then FRU is absent.

Signed-off-by: Sridevi Ramesh <sridevra@in.ibm.com>
Change-Id: I5534d6cc1a9f7a1b329975cdd5de54110f456f39