linux-yocto: Add support for building from YOCTO and GITHUB private repo

- Add support for building also from YOCTO repos and from the tip of
the LSI GitHub private repo depending what .inc files is included.
This choice is made by settingthe 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>