[dovetail] Bugfix: incorrectly exit when file doesn't exist

When checking files, it shouldn't exit when getting an non-zero results.
However, the scripts set -e at the begining.
In order to continue the following steps when the file doesn't exists, replace
all "return 0" or "return 1" to be "echo 0/1".

Change-Id: I28ea81d52182d4dc5db56d47abbfbc1761f17d81
Signed-off-by: xudan <xudan16@huawei.com>