ohos-image.bbclass: Copy boot assets to root filesystem

This implements logic as a rootfs postprocess command where the set of
boot artifacts are provided as part of the root filesystem. The reason
is that the system update components assume only the root filesystem as
part of the update payload while boot files are also needed depending on
the BSP. For example, RaspberryPi update process would update the kernel
and dtbs on the boot partition too.

The boot files are copied to the path defined by
ROOTFS_BOOT_ARTIFACTS_PATH, relative to the root of the root filesystem.

SOme of the logic in this mechanism are based on the current
implementation of the bootimg-partition wic plugin.

Fixes https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/issues/104



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