linux-yocto: introduce 4.15 recipes

linux-yocto: introduce 4.15 recipes

This commit makes the 4.15 kernel available for use with the
Yocto configuration fragments and qemu* BSPs.

It has been tested for x86,arm,mips and powerpc against the lsb, core*
and glibc/mulsc test matrix.

This will serve as the "latest" kernel in master, with others being
removed in subsequent commits.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>