busybox: add config fragments

busybox: add config fragments

Add config fragments to busybox.

The implementation makes use of merge_config.sh script in kern-tools-native.
The use case is similar to the yocto kernel's configuration fragments.

We also add kern-tools-native to busybox's DEPENDS variable to ensure
that merge_config.sh is available when required.

[YOCTO #3379]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>