sstate: Fix dir ownership issues in SSTATE_DIR

sstate: Fix dir ownership issues in SSTATE_DIR

We currently use mkdir -p to create missing parent directories within SSTATE_DIR.
Reading the man page for mkdir mentions that parent directories are created with
the current umask, *not* the mode passed upon the commandline.

We could fix this by setting and resetting the umask but since we already have
decent python code able to do this, move to using that injecting a python function
into the chain of functions already present.

This should help fix the occasional sstate directory creation with the wrong
permissions.

[YOCTO #14385]

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