ANDROID: db845c: Use FRAGMENT_CONFIG

./build/config.sh provides Kbuild's configuration editor in kernel/build
environment, plus support for editing config fragments. Set
FRAGMENT_CONFIG for ./build/config.sh.

Bug: 169398826
Signed-off-by: Elliot Berman <eberman@codeaurora.org>
[jstultz: Dropped sorting the config fragment]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: Id09967bb06d0652788d17373ade2619a310c9e87