Modernize the MAKE_JUST_KERNELS hint in the top-level makefile.

It doesn't make sense to limit to -j12 anymore, build scalability
is better than it used to be.  Fold the hint into the description
of the universe target.

Reviewed by:	imp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D20342

Makefile

@@ -4,7 +4,8 @@
 # The user-driven targets are:
 #
 # universe            - *Really* build *everything* (buildworld and
-#                       all kernels on all architectures).
+#                       all kernels on all architectures).  Define the
+#                       MAKE_JUST_KERNELS variable to only build kernels.
 # tinderbox           - Same as universe, but presents a list of failed build
 #                       targets and exits with an error if there were any.
 # buildworld          - Rebuild *everything*, including glue to help do
@@ -45,12 +46,6 @@
 # native-xtools-install
 #                     - Install the files to the given DESTDIR/NXTP where
 #                       NXTP defaults to /nxb-bin.
-# 
-# "quick" way to test all kernel builds:
-# 	_jflag=`sysctl -n hw.ncpu`
-# 	_jflag=$(($_jflag * 2))
-# 	[ $_jflag -gt 12 ] && _jflag=12
-# 	make universe -DMAKE_JUST_KERNELS JFLAG=-j${_jflag}
 #
 # This makefile is simple by design. The FreeBSD make automatically reads
 # the /usr/share/mk/sys.mk unless the -m argument is specified on the