kernel-yocto.bbclass: Fix deps when externalsrc is used

do_kernel_configme was recently removed from SRCTREECOVEREDTASKS so this
task still runs when externalsrc is used. This task normally runs after
do_patch but when externalsrc is used, do_patch is removed and this ordering
restriction does nothing. This allows bitbake to execute do_kernel_configme
too early, causing races with do_unpack.

This is fixed by adding in a dependency on do_unpack when externalsrc is
used.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>