sstate: Ensure a given machine only removes things which it created
Currently if you build qemux86 and then generic86, the latter will
remove all of the former from deploy and workdir. This is because
qemux86 is i586, genericx86 is i686 and the architctures are compatible
therefore the sstate 'cleaup' code kicks in.
There was a valid reason for this to ensure i586 packages didn't get into
an i686 rootfs for example. With the rootfs creation being filtered now, this
is no longer necessary.
Instead, save out a list of stamps which a give machine has ever seen in
a given build and only clean up these things if they're no longer
"reachable".
In particular this means the autobuilder should no longer spend a load of time
deleting files when switching MACHINE, improving build times.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
