Target build output, stored in do_populate_sysroot or do_package can depend
not only upon direct dependencies but also indirect ones. A good example is
linux-libc-headers. The toolchain depends on this but most target recipes do
not. There are some headers which are not used by the toolchain build and do
not change the toolchain task output, hence the task hashes can change without
changing the sysroot output of that recipe yet they can influence others.
A specific example is rtc.h which can change rtcwake.c in util-linux but is not
used in the glibc or gcc build. To account for this, we need to account for the
populate_sysroot hashes in the task output hashes.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
