unified: Fix boot_time nanokernel benchmark

Enables boot time timestamps for unified kernel.

Also Splits the source code into microkernel and nanokernel versions
instead of having common code. Not only does this make the code for
each project easier to read, but it also easily allows the nanokernel
version to link against the correct version of main().

Change-Id: Ie0afa2272c3347ebdacc0e3daeebbfe9583fe596
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>