Add SBT_FLAGS Make variable to SBT definition.

In preparation for support of build-specific chisel versions (for
Jenkins HEAD builds), add the (undefined) Make variable SBT_FLAGS to
the definition of SBT.
Support optional command-line chiselVersion

Add support for command-line definition of chiselVersion (default is
"latest.release"), and use 2.3-SNAPSHOT for new jenkins-build target.