Makefile:  replace objdir path names with $(OBJDIR)
Add BUILD_THREAD_FLAVORS list of all configured RTOS systems

Be sure make threads=<flavor> is a valid, configured flavor

Clean up all thread systems build artifacts with 'make clean', not
just the $(threads) build system
Pull KBUILD_EXTRA_SYMBOLS into configure.in; fix RTAI build
Compile ulapi.so sources separately for each flavor

ULAPI_SRCS contain flavor-specific macros and need to be built
separately for each flavor.

This is a bit of a hack since USERSRCS don't provide any facility for
this.
enable shmdrv by default
Move #includes out of rtai-kernel.h into rtai-kernel.c

This is the proper scope for these declarations, and they're needed by
the separated ulapi.so compilation.
rtapi/.gitignore:  ignore kthreads flavors too
Fix broken LDFLAGS for userland rtapi.so modules

Hmm, not sure why this ever worked....
Makefile:  build all configure flavors if none specified