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 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>
linux-yocto: Add support for building from GITHUB public repo

- Add support for building also from the LSI GitHub public repo depending
what .inc files is included. This choice is done by setting the LSI_SRC
variable in the build's local.conf file.
- By default, with LSI_SRC not set in local.conf, the Kernel will be
fetched from YOCTO repos. This is done because LSI_SRC is set to
"linux-yocto" in receipes. local.conf will overwrite it.
LSI_SRC ?= "linux-yocto"

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