flash_ec: add support for flashing with Toad cable
flash_ec: add support for flashing with Toad cable

Add support for flashing Spring board through the Toad cable (given the
Write Protect screw is not on).

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BRANCH=spring
BUG=chrome-os-partner:17570
TEST=with both a Toad cable and a servo v2, flash EC on Spring EVT
with the following command:
./util/flash_ec --board=spring
and check the state of the servo/toad before and after.

Change-Id: Ia4e0d32b062d58b4e906d3f006003fa6097add83
Reviewed-on: https://gerrit.chromium.org/gerrit/48031
Reviewed-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

longsleep/ec,fourier49/BZ_DEV_EC,alterapraxisptyltd/chromium-ec,fourier49/BZ_DEV_EC,mtk09422/chromiumos-platform-ec,fourier49/BZ_DEV_EC,thehobn/ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,alterapraxisptyltd/chromium-ec,coreboot/chrome-ec,eatbyte/chromium-ec,mtk09422/chromiumos-platform-ec,longsleep/ec,thehobn/ec,coreboot/chrome-ec,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,longsleep/ec,gelraen/cros-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,gelraen/cros-ec,alterapraxisptyltd/chromium-ec,eatbyte/chromium-ec,md5555/ec,gelraen/cros-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,gelraen/cros-ec,longsleep/ec,fourier49/BIZ_EC,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,thehobn/ec,thehobn/ec,eatbyte/chromium-ec,coreboot/chrome-ec,md5555/ec,fourier49/BIZ_EC,fourier49/BIZ_EC,alterapraxisptyltd/chromium-ec,md5555/ec