sstate: Fix rebuilds when changing layer config
When adding a layer which changed SSTATE_EXCLUDEDEPS_SYSROOT, the state
hashes were changing when they should not. This was caused by wider use
of setscene_depvalid which means the dependency on the variable was seen
when it was previously not.
Exclude the variable since this should be be included in the hashes.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit
09725a29365c69ccbd603fe3a1de72189f26d5ac)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
