kernel-yocto: clarify KMETA branch comments

kernel-yocto: clarify KMETA branch comments

Having a meta branch is not strictly required by the tools (and
recipes like linux-yocto-custom do not have meta branches), but the
comments in the kernel-yocto.bbclass could lead someone to think that
it was required.

This commit clearifies the comment to the following:

  # We can fix up the kernel repository even if it wasn't a bare clone.
  # If KMETA is defined, the branch must exist, but a machine branch
  # can be missing since it may be created later by the tools.

[YOCTO #3422]

(From OE-Core rev: 421a2e2523a8f2c461479a1c0d44908cc1eaca6b)

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