package/linux-headers: add support for CIP kernel versions with same-as-kernel

When the linux-headers are configured to use the same source as the
kernel (BR2_KERNEL_HEADERS_AS_KERNEL), and the kernel is configured
to be one of the two CIP versions (BR2_LINUX_KERNEL_LATEST_CIP_VERSION
or BR2_LINUX_KERNEL_LATEST_CIP_RT_VERSION), the build fails if the
kernel sources are not already downloaded:

    $ cat defconfig
    BR2_LINUX_KERNEL=y
    BR2_LINUX_KERNEL_LATEST_CIP_VERSION=y
    $ make defconfig BR2_DEFCONFIG=$pwd)/defconfig
    $ make linux-headers-source

    >>> linux-headers 4.19.118-cip25 Downloading
    --2020-05-13 19:28:44--  https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.19.118-cip25.tar.xz


    Resolving cdn.kernel.org (cdn.kernel.org)... 2a04:4e42:1d::432, 151.101.121.176
    Connecting to cdn.kernel.org (cdn.kernel.org)|2a04:4e42:1d::432|:443... connected.
    HTTP request sent, awaiting response... 404 Not Found
    2020-05-13 19:28:45 ERROR 404: Not Found.

    make[1]: *** [package/pkg-generic.mk:171: /home/ymorin/dev/buildroot/O/build/linux-headers-4.19.118-cip25/.stamp_downloaded] Error 1
    make: *** [Makefile:23: _all] Error 2

We fix that by adding yet another duplication of information out of
the linux.mk, to use the CIP-specific git tree where to get the
archives as snapshots.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>