unified/build: allow building the unified kernel

Added needed kconfig options. KERNEL_V2 selects MICROKERNEL to allow
middleware and application that differentiate between NANOKERNEL and
MICROKERNEL to run unmodified.

Build the unified/ kernel directory: do not touch the
nanokernel/microkernel directories.

Invoke sysgen for both microkernel and unified kernel. Only have sysgen
reference include/microkernel if building an original microkernel.

Change-Id: If74779146143434f7ee274bbef32d6c894b9f1a1
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>