Add make universe targets "kernels" and "worlds".

"make kernels" is now shorthand for "make universe -DMAKE_JUST_KERNELS"
"make worlds" is now shorthand for "make universe -DMAKE_JUST_WORLDS"

The kernels target includes modules (unless you add -DNO_MODULES).

And of course you can still add all the other universe options, such as
"make kernels TARGETS=arm" to build kernels for all arm arches, or
TARGET_ARCH=armv6 to build all armv6 kernels, etc.

Reviewed by: imp