linux-yocto: Add support for building from the tip of private repo

- Add support for building also from the tip of the private repo by
depending what .inc files is included. This choice is made by setting
the LSI_SRC variable in the build's local.conf file.
- Add FIT support files to SRC_URI for preempt-rt kernels.

Signed-off-by: Paul Butler <paul.butler@windriver.com>