rootfs: Add debugfs package db file copy and cleanup

When copying the package database files for the debugfs, add individual
file copy as well as tree copying. After the debug rootfs has been
created, cleanup the package files.

This then allows us to avoid a problem where (for rpm at least)
extraneous files in the debug rootfs would cause failures during
oe-selftest because some files existed in both regular and debugfs
images.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>