staging: Remove uninstalled dependencies from sysroots
Currently, if something is added to a sysroot, its hash remains unchanged,
and it continues to be buildable, it doesn't get removed from the sysroot.
This patch handles the case where something is removed from DEPENDS or
[depends].
It does introduce its own issue where something could get removed even
though some other task in parallel may have the same requirement. This
case should be extrememly rare and fixing the more common DEPENDS removal
is likely the bigger win though.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
