buildhistory: add missing vardepsexcludes

For POPULATE_SDK_POST_TARGET_COMMAND, POPULATE_SDK_POST_HOST_COMMAND, and SDK_POSTPROCESS_COMMAND, the appropriate entries were added to vardepvalueexclude, but we want them in vardepsexclude as well.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 554b17e0)
Signed-off-by: Steve Sakoman <steve@sakoman.com>