fastboot: Move conditional inclusion to top-level Makefile

Instead of adding depthcharge-$(CONFIG_FASTBOOT_MODE) to every single
file under fastboot/, include fastboot directory in subdirs- only if
CONFIG_FASTBOOT_MODE is y.

BUG=None
BRANCH=None
TEST=Compiles successfully both with and without fastboot flag.

Change-Id: Id6da208f0e25f5e610e48e73a5f6499ca24ebc9c
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://chromium-review.googlesource.com/265922
Trybot-Ready: Furquan Shaikh <furquan@chromium.org>
Tested-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Commit-Queue: Furquan Shaikh <furquan@chromium.org>
2 files changed