kernel: restore scripts in the sysroot

kernel: restore scripts in the sysroot

When building against the sysroot, out of tree modules can require modpost
and other utilities normally found in the kernel's scripts directory. For
the kernel source in the staging dir, these scripts have been removed to
avoid mixing archiectures when packaging kernel-dev (among other things).

Rather than further complicate the kernel's install rule, or its packaging,
we can restore the scripts by building them in the kernel staging directory
after the sstate is installed, making them available to packages that need them.

(From OE-Core rev: 5bcd65807aa634060f98928db6011856934dabe4)

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