ohos-image.bbclass: Avoid duplicates in install_task when deploying boot assets

The code detects when boot assets would overwrite something already
deployed/installed in the rootfs. This helps to avoid breaking packaging
at runtime. On the other hand, if IMAGE_BOOT_FILES has duplicates, this
check will fail when trying to install the same file multiple times. To
avoid this, treat the install_task as a set.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>