build.mk: Define a constant for OSv kernel builds

It is sometimes useful to programatically learn if something will end up
in the main ELF or in a shared library. This is the function usually
performed by _KERNEL. However, using that proved quite difficult when
Asias was trying to conditionally compile some zfs tools.  This is
because a lot of our headers expect _KERNEL to be defined and they are
also included in the tools build. Too messy, unfortunately.

Because of that, I am defining a new constant that will be available for
every object that will end up in the main ELF, but won't be defined
otherwise.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>