sstate.bbclass: Add _SSTATE_EXCLUDEDEPS_SYSROOT to vardepsexclude

sstate.bbclass: Add _SSTATE_EXCLUDEDEPS_SYSROOT to vardepsexclude

When using tinfoil to control the build, multiple commands (serially) could
trigger an error such as:

  When reparsing ....bb:do_package, the basehash value changed from ... to .... The metadata is not deterministic and this needs to be fixed.
  ERROR: The following commands may help:
  ERROR: $ bitbake esw-conf -cdo_package -Snone
  ERROR: Then:
  ERROR: $ bitbake esw-conf -cdo_package -Sprintdiff

However following these commands it was not able to be reproduced.  Forcing
bitbake to dump the signatures and then running bitbake-diffsigs showed
that the value of _SSTATE_EXCLUDEDEPS_SYSROOT was being set in one run, but
was blank is a different version.

Upon inspecting the code in sstate.bbclass, one usage (without the _) is
already excludes, the leading _ version is used as a cache, only if set but
is not actually required to be defined.  So ignoring the value should work
properly.

Signed-off-by: Mark Hatle <mark.hatle@amd.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>