then that target is invoked for each architecture rather than the
default action of building world and kernels for each architecture.
- Add a 'make toolchains' wrapper which uses UNIVERSE_TARGET to build
toolchains for all architectures.
- Document JFLAG, MAKE_JUST_KERNELS, and MAKE_JUST_WORLDS variables for
'make universe'.
Reviewed by: bz
MFC after: 1 week