Simplify and clean up "update" target. Also

Simplify and clean up "update" target. Also
add an SVN-UPDATE-OPTIONS variable that controls
the update behaviour. This allows to go to a common
older revision of all svn directories:

  gmake update SVN-UPDATE-OPTIONS=-r47717

would rewind both llvm and clang to a common
revision (if clang is checked out into the llvm
tree).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47717 91177308-0d34-0410-b5e6-96231b3b80d8