This patch adds support for EC EFS: EC verifies and jumps to RW
image before AP boots.

Depthcharge refers the active slot for checking RW version and
the update slot for update (a.k.a. soft sync).

This change removes reboot_all_ecs and adds cros_ec_reboot, which
takes EC_REBOOT_FLAG_*. Other ECs are expected to be rebooted
when the main EC reboots the whole system.

BUG=b:65028930,b:65264494
BRANCH=none
CQ-DEPEND=CL:616346
TEST=On Fizz, verify:
1. RW_B is old and updated by soft sync. RW_B is activated and
   executed after reboot. System continues to boot to OS.
2. RW_A is old and updated by soft sync. RW_A is activated and
   executed after reboot. System continues to boot to OS.

Change-Id: Ida61e51ab5e61376f836c9d8027f7f21eb205ace
Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/616248