Add an additional layer of inheritance for tegu products.
This change adds a common layer of inheritance for non-factory
products (tegu_generic.mk). This makes it easier to configure
factory and non-factory products differently.
Non-factory `tegu` products now inherits from
`device/google/tegu/tegu_generic.mk` which inherit from
`device/google/tegu/device-tegu.mk`.
FLAG: EXEMPT mechanical refactoring
Bug: 336452008
Bug: 345198819
Test: lunch tegu-trunk_staging-userdebug && m
Change-Id: I5a48fe09f825ec7d8dff50ff36b1c1b1b1a6fa22
