kselftests-next: rename Git repository to kernel-next
kselftests-next: rename Git repository to kernel-next

It may happen that if a version is set for a kernel (not from
linux-next), the fetcher will be unable to find the specificed
revision in the tree. Exempli gratia:

If SRCREV_kernel is set to:
  0f105cf4f60e5afdf2932fed7f05ce776ce14289
globally, (which is a fine kernel for torvalds/master,
mainline), then this package would try to fetch the very same
revision but from linux-next/master:
  WARNING: kselftests-next-4.15+gitAUTOINC+0f105cf4f6-r0 do_fetch: Failed to fetch URL git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git;protocol=https;branch=master;name=kernel, attempting MIRRORS if available
  ERROR: kselftests-next-4.15+gitAUTOINC+0f105cf4f6-r0 do_fetch: Fetcher failure: Unable to find revision 0f105cf4f60e5afdf2932fed7f05ce776ce14289 in branch master even from upstream
  ERROR: kselftests-next-4.15+gitAUTOINC+0f105cf4f6-r0 do_fetch: Fetcher failure for URL: 'git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git;protocol=https;branch=master;name=kernel'. Unable to fetch URL from any source.
  ERROR: kselftests-next-4.15+gitAUTOINC+0f105cf4f6-r0 do_fetch: Function failed: base_do_fetch

If we want to accomodate two different versions of the Linux
kernel and kselftests (or rather different repositories)
then we need to have this renamed here.

Signed-off-by: Daniel Díaz <81c1e6f46dfb75ebff90afb11720614d43e597b3@linaro.org>

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