When building multiple kernels using KERNCONF, non-existent KERNCONF

files will produce an error and buildkernel will fail. Previously 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.

This is a rework of r302865.

This is the correct patch.

Reviewed by:	ngie (previous version, r302865)
MFC after:	2 months
Differential Revision:	D7167

Makefile.inc1

@@ -1359,6 +1359,10 @@ BUILDKERNELS+=	${_kernel}
 .if empty(INSTALLKERNEL) && !defined(NO_INSTALLKERNEL)
 INSTALLKERNEL= ${_kernel}
 .endif
+.else
+.if make(buildkernel)
+.error Missing KERNCONF ${KERNCONFDIR}/${_kernel}
+.endif
 .endif
 .endfor
 

UPDATING

@@ -51,6 +51,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 12.x IS SLOW:
 
 ****************************** SPECIAL WARNING: ******************************
 
++20160714:
+	When building multiple kernels using KERNCONF, non-existent KERNCONF
+	files will produce an error and buildkernel will fail. Previously
+	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.
+
 20170912:
 	The default serial number format for CTL LUNs has changed.  This will
 	affect users who use /dev/diskid/* device nodes, or whose FibreChannel