inventec: layer.conf: Add INVENTECBASE

Define a variable that recipes can use for finding the root
of the inventec layer.  Typically this is used to refer to the
layer LICENSE for recipes that don't have any source repository.

(From meta-inventec rev: 169daaa946cd053ddb7cc5514fb178497cd9cb30)

Change-Id: If785f8eb05d56cc9653d5924c0c4512cf681d1cd
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>