futility: cmd_update: Correctly handle error counter
futility: cmd_update: Correctly handle error counter

The errorcnt in do_update should not add function return values
directly because the function may return negative values, which would
lead to wrong results. Instead we can process using '!!' so the returned
value will always be zero or positive integers.

BUG=chromium:875551
TEST=make futil; tests/futility/run_test_scripts.sh $(pwd)/build/futility
BRANCH=None

Change-Id: I9f450b2ee8d86035288f06bdb314dfb1dce3ca64
Signed-off-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1209023
Reviewed-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot