package: don't use BR2_KERNEL_MIRROR for git downloads

The git repositories are not served on the kernel.org CDN:

    fatal: repository 'https://cdn.kernel.org/pub/scm/linux/kernel/git/will/kvmtool.git/

' not found

Switch to explicitly use the git.kernel.org server.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>