build: fix restoring /etc/opkg with PER_DEVICE_ROOTFS

build: fix restoring /etc/opkg with PER_DEVICE_ROOTFS

When generating per-device rootfs directories, the ./etc/opkg/ directory
is moved away prior to calling opkg install, opkg remove and rootfs_prepare.
After the opkg invocations and the rootfs_prepare macro call, the saved opkg
config directory is supposed to be moved back to its previous ./etc/opkg
location.

The mv command however can fail to properly restore the directory under
certain circumstances, e.g. when the prior opkg or files/ overlay copy
operations caused a new ./etc/opkg/ directory to be created.

In this case, the backed up directory (named target-dir-$hash.opkg) will be
moved into the preexisting ./etc/opkg/ directory instead, causing the opkg
configuration to be located in a wrong path on the final rootfs, e.g. in
/etc/opkg/target-dir-$hash.opkg/distfeeds.conf instead of
/etc/opkg/distfeeds.conf.

Solve this problem by replacing the naive "mv" command with a recursive
"cp -T" invocation which causes the backed up directory tree to get merged
with the destination directory in case it already exists.

Also perform the rootfs_prepare macro call after restoring the opkg
configuration, to allow users to override it again by using the files/
overlay mechanism.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>