unified/build: adapt Kbuild for unified kernel

Changes needed to build system to build a unified kernel:

- NUM_COMMAND_PACKETS, NUM_TIMER_PACKETS, NUM_TASK_PRIRITIES must be
  passed to sysgen to avoid modifying it. Simply set them to 0. The
  unified build does not need them.

- Pass a different kernel type to sysgen if building for a unified
  kernel.

- Build kernel_main.c if building an MDEF file (micro/unified).

- The location of the include files for building offsets.h is different
  for unified kernels and nano/micro kernels.

Change-Id: I46b30a2b5ffc3ca675b3413150643b92c117c2c8
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>