In looking at some of the reproducibility failures, I realised that
several are from a common cause, basically tasks missing a umask
setting. Our common tasks have this but for example:

kernel-devsrc - copies files from ${S} which are created in special
kernel tasks which don't have umask set

quilt-ptest - contains files created by do_compile_ptest_base which has
no umask set.

Looking at the output for valgrind-ptest I suspect something similar.

We could continue to try and fix this per task, or we could set a
default umask for tasks. I was surprised that bitbake didn't have an
option for that so I added one, BB_DEFAULT_UMASK.

I'll send out patches for bitbake and OE-Core, I think/hope this change
shouldn't be controversial but I wanted people to be aware of the
change.

I'm using a value of 022 since that is what is used for most tasks
currently. We can get rid of quite a few umask variable flag settings
after this which is a nice cleanup.

Cheers,

Richard