util/flash_ec: Fix shellcheck warning
util/flash_ec: Fix shellcheck warning

DEFINE_string board "${DEFAULT_BOARD}" \
		     ^--------------^ SC2154: DEFAULT_BOARD is
				      referenced but not assigned.

The DEFAULT_BOARD variable is never set, so replace it with an empty
string and print an error if the board is not specified.

BRANCH=none
BUG=b:242127759
TEST=./util/flash_ec --board=dartmonkey

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I433bbd6105debc2494fc93f8987e6fe271ef93d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3864975
Reviewed-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec