Revert r302865 and it's sister r302866 as r302865 causes some breakage.

Reported by:	Guy Yur <guyyur@gmail.com>

Makefile.inc1

@@ -1140,8 +1140,6 @@ BUILDKERNELS+=	${_kernel}
 .if empty(INSTALLKERNEL) && !defined(NO_INSTALLKERNEL)
 INSTALLKERNEL= ${_kernel}
 .endif
-.else
-.error Missing KERNCONF ${KERNCONFDIR}/${_kernel}
 .endif
 .endfor
 

UPDATING

@@ -31,14 +31,6 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 12.x IS SLOW:
 	disable the most expensive debugging functionality run
 	"ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
-20160714:
-	As of r302865 when building multiple kernels using KERNCONF,
-	non-existent KERNCONF files will produce an error and buildkernel
-	will fail. Previous to r302865 missing KERNCONF files silently
-	failed giving no indication as to why, only to subsequently discover
-	during installkernel that the desired kernel was never built in the
-	first place.
-
 20160622:
 	The libc stub for the pipe(2) system call has been replaced with
 	a wrapper which calls the pipe2(2) system call and the pipe(2) is now