bitbake: bitbake: Share BB_TASKDEPDATA with tasks

bitbake: bitbake: Share BB_TASKDEPDATA with tasks

Currently tasks have no knowledge of which other tasks they depend
upon. This makes it impossible to do at least two things which would be
desirable/interesting:

a) Have the ability to create per recipe sysroots
b) Allow the aclocal files to be present only for the entries in
   DEPENDS (directly and indirectly)

By exporting task data through this new variable, tasks can inspect
their dependencies and then take actions based upon this.

(Bitbake rev: 84f1dde717dac22435005b79d03ee0b80a3e8e62)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>