flash_ec: Allow dfu updates without servo console
flash_ec: Allow dfu updates without servo console

flash_ec is used for both ec chips accessed over servo, and
standalone stm32 devices. It's not necessary to have a servo
conencted to update the standalone devices over usb.

This is useful for servo v4 and servo micro.

BUG=chromium:571477
TEST=Verify servo micro/discovery can be flashed without servo v2.
BRANCH=none

Signed-off-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>

Change-Id: I9deee1616d93feeac4d6675bc3a4f573d4906f7b
Reviewed-on: https://chromium-review.googlesource.com/321925
Commit-Ready: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>
Tested-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>
Tested-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@google.com>
Reviewed-by: Wai-Hong Tam <04b587fdf5845741a0c5a8b9cd59ca72d73ef8fc@chromium.org>
Reviewed-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec