ts-kernel-build: enable CONFIG_IKCONFIG{_PROC}

ts-kernel-build: enable CONFIG_IKCONFIG{_PROC}

This makes the kernel's .config available in /proc/config.gz and
embeds a copy which can be extracted with
linux/scripts/extract-ikconfig (which I've not tried, but have no
reason to doubt).

Having this around can be handy with an older osstest installed test
box and to confirm you've booted the kernel you think you have when
you are messing with .config options.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>