kernel-yocto/linux-yocto-custom: support low bandwidth options

kernel-yocto/linux-yocto-custom: support low bandwidth options

To support configurations where active development is not being done within
the oe/bitbake build environment and restricted bandwidth situations, this
commit allows the SRC_URI to point to a kernel tgz instead of a full git
repository.

Outside of the upstream tgz instead of a kernel git repository, the
restrictions, config and patch process is the same as any linux-yocto-custom
recipe.

An example linux-yocto-custom based recipe would have a configuration like
this to build the 3.7 kernel, using an externally supplied config, from the
3.7 tgz:

  SRC_URI = "http://kernel.org/pub/linux/kernel/v3.0/linux-3.7.tar.bz2"
  PV = "3.7"
  S = "${WORKDIR}/linux-3.7"
  SRC_URI[md5sum] = "5323f3faadd051e83af605a63be5ea2e"
  SRC_URI[sha256sum] = "dc08d87a579fe2918362e6666e503a95a76296419195cb499aa9dd4dbe171a9e"

[YOCTO #2686]

(From OE-Core rev: 08b3a282ce75a9972694f0c4379179505b9ec91f)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>