support/scripts/graph-depends: use proper rootpkg in remove_extra_deps()

support/scripts/graph-depends: use proper rootpkg in remove_extra_deps()

The remove_extra_deps() function removes dependencies that we are not
interested in seeing in the dependency graph. It does this for all
packages, except the 'all' package, which on full dependency graphs is
the root of the tree.

However, this doesn't take into account package-specific dependency
graphs (i.e make <pkg>-graph-depends) where the root is not 'all', but
'<pkg>'. Due to this, dependencies on "mandatory deps" were not
visible at all, i.e the toolchain package (and its dependencies) and
the skeleton package (and its dependencies) were not displayed in
package-specific dependency graphs.

To fix this, we use the existing rootpkg variable instead of
hardcoding 'all'.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>