[OE-core] reproducible_final_image_task failing when REPRODUCIBLE_TIMESTAMP_ROOTFS unset

[Alex Kiernan]

I'm trying to use BUILD_REPRODUCIBLE_BINARIES, but I tripped over a
problem when I'd not set REPRODUCIBLE_TIMESTAMP_ROOTFS.

In the reproducible_final_image_task (), we try to set
REPRODUCIBLE_TIMESTAMP_ROOTFS based on the top most git timestamp, but
that's failing because the working directory at that point is my build
directory, which isn't git controlled:

| NOTE: Executing reproducible_final_image_task ...
| DEBUG: Executing shell function reproducible_final_image_task
| fatal: Not a git repository (or any of the parent directories): .git
| WARNING: /home/vagrant/poky-master/build/tmp/work/nanohub-poky-linux-gnueabi/nano-image-core/1.0-r0/temp/run.reproducible_final_image_task.3664:1
exit 128 from 'REPRODUCIBLE_TIMESTAMP_ROOTFS=`git log -1
--pretty=%ct`'

Clearly just setting REPRODUCIBLE_TIMESTAMP_ROOTFS in my global config
fixes the problem, but that doesn't help anyone else trying to use
this... I'm really not sure what repo that piece of code is intending
to target.

-- 
Alex Kiernan