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:
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@47717 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,dslab-epfl/asap,llvm-mirror/llvm,chubbymaggie/asap,apple/swift-llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,chubbymaggie/asap,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,chubbymaggie/asap,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm