Add uart_prefix flag to control which uart is used to flash.
Add uart_prefix flag to control which uart is used to flash.

There are CrOS devices that have multiple embedded controllers and
therefore multiple uarts that can be used for programming.

This CL allows user to set the uart_prefix to access the alternate
uarts via the --uart_prefix argument.  Default is still 'ec'.

Signed-off-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

BRANCH=none
BUG=chrome-os-partner:28826
TEST=util/flash_ec --board=samus_pd --uart_prefix=usbpd

Change-Id: I9fbe8d13067b7f514447645b2587dda706445661
Reviewed-on: https://chromium-review.googlesource.com/199900
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Commit-Queue: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

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