python tasks calling shell functions using exec_func() would show the log
file as /dev/null. It makes most sense for all the task logging to be setup
centrally by exec_task(), at least with the current code base in Poky.
This commit will need discussion in relation to upstream bitbake and the
IO redirection could be better handled using a context manager (although
task contexts shouldn't ever nest).
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
