image.bbclass/rootfs.py: add variables to rootfs[vardeps]
Added base variables and package backend specific variables to
rootfs[vardeps] in order for rootfs to rebuild when changes are made.
Set some variables as [func] to inform bitbake that they are shell
scripts, so that it invokes its shell dependency parsing. Without
marking them as functions, changes in the actual function body would
not trigger rootfs rebuilds.
[YOCTO #6502]
Signed-off-by: Roxana Ciobanu <roxana.ciobanu@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
