lineage: Move some kernel definitions to BoardConfigKernel

Kernel source settings should always come at BoardConfig population
time so things that use the variable later don't end up pointing to
an empty or wrong variable.

The following is also squashed in:

Author: Christopher N. Hesse <raymanfx@gmail.com>
Date:   Fri Aug 10 00:23:54 2018 +0200
    tasks: kernel: Honor prebuilt kernel flag
    For devices that want to use a prebuilt kernel, TARGET_KERNEL_SOURCE
    would still be set to TARGET_AUTO_KDIR, meaning the build system would
    still try to build the kernel if TARGET_AUTO_KDIR was present.
    Setting TARGET_PREBUILT_KERNEL indicates this is not wanted, so don't
    attempt to do it.
    Change-Id: Ic79b3ac1b9c946fd258ada43dce2b08bb74ea0d9

Change-Id: If046b86ff0d18c76898e90295be873a8379f678a