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>

Add it to handle recipe-depends.dot and task-depends.dot. E.g.:

* Print why rpm is built
  $ oe-depends-dot -k rpm --why/-w recipe-depends.dot
  Because: core-image-sato libdnf libsolv dnf

* Print bzip2-native's depends
  $ oe-depends-dot -k bzip2-native --depends/-d recipe-depends.dot
  Depends: automake-native gnu-config-native libtool-native quilt-native autoconf-native

* Remove duplicated dependencies to reduce the size of the dot files.
  For example, A->B, B->C, A->C, then A->C can be removed. The dot files are too
  big, we nearly couldn't use 'dot -T' to generate pictcures for target recipes,
  remove the duplicated dependencies makes is it possible.
  $ bitbake core-image-sato -g
  $ oe-depends-dot -r recipe-depends.dot
  Saving reduced dot file to recipe-depends-reduced.dot
  $ du -sh recipe-depends*.dot
  608K    recipe-depends.dot
  32K     recipe-depends-reduced.dot

  It has been recuded from 608K to 32K, now we can generate a picture,
  otherwise, it is too big:
  $ dot -Tpng recipe-depends-reduced.dot -O

It also can handle task-depends.dot.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>