We have other places in the code where we need to take filemode/mask
information from a bitbake variable and turn it into a real python
number. Turn this internal code into public API in bb.utils and
add some tests for it.
(Bitbake rev: d89e30fb2fb15b09f2cb95c4e5aa9f749ca257ea)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>