sstate: Fix rebuilds when changing layer config

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>