kernel: Support building from kernel platform

This expects a few build variables to be set:

* TARGET_KERNEL_PLATFORM_TARGET: The base target name. For example,
  'tegra' will indicate that the dist target is tegra_dist, abi is
  tegra_abi, etc.
* TARGET_KERNEL_VERSION: Used to determine the kernel checkout path. For
  example, a value of 6.6 will expect the kernel platform repo to be
  checked out to $BUILD_TOP/../kernel-6.6.
* TARGET_KERNEL_SOURCE: Relative to the top of the kernel platform
  checkout, the path to the kernel build rules. For example,
  vendor/nvidia/tegra.
* BOARD_KERNEL_IMAGE_NAME: The kernel image to use. For example, Image.

Using the examples above, this will run bazel in the kernel-6.6
directory on the target //vendor/nvidia/tegra:tegra_dist, then source
the output from $OUT/tegra/dist, with the prebuilt kernel image being
Image.

Change-Id: If96e96dffa2c0bfd9bd824bb3d8064eb861c9d00
4 files changed