oe-depends-dot: print dependency chains for '--why' option

oe-depends-dot: print dependency chains for '--why' option

When using '--why' option, we currently only list elements.
It's better to print out dependency chains. This patch adds
such abitility.

e.g.
  $ oe-depends-dot -k util-linux -w recipe-depends.dot
  Because: packagegroup-core-boot systemd-compat-units systemd shadow core-image-minimal dbus e2fsprogs
  core-image-minimal -> packagegroup-core-boot -> systemd-compat-units -> systemd -> dbus -> shadow -> util-linux
  core-image-minimal -> packagegroup-core-boot -> systemd-compat-units -> systemd -> dbus -> e2fsprogs -> util-linux

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>