In kern.pre.mk, split SYSTEM_LD into two variables to avoid duplication in

other files.

Arm and mips systems need to replace the SYSTEM_LD variable because they
need to create intermediate files which are post-processed with objcopy to
create the final .TARGET file. Previously they did so by pasting the full
expansion of SYSTEM_LD with the output filename replaced. This means
changing SYSTEM_LD in kern.pre.mk means you need to chase down anything that
replaces it and figure out how it differs so you can paste your changes in
there too.

Now there is a SYSTEM_LD_BASECMD variable that holds the entire basic kernel
linker command without the input and output files. This will allow arm and
mips makefiles to create their custom versions by refering to
SYSTEM_LD_BASECMD, which then becomes the one place where you have to make
changes to the basic linker command args.

Differential Revision:	https://reviews.freebsd.org/D22921

sys/conf/kern.pre.mk

@@ -266,10 +266,11 @@ MD_ROOT_SIZE_CONFIGURED!=	grep MD_ROOT_SIZE opt_md.h || true ; echo
 SYSTEM_OBJS+= embedfs_${MFS_IMAGE:T:R}.o
 .endif
 .endif
-SYSTEM_LD= @${LD} -m ${LD_EMULATION} -Bdynamic -T ${LDSCRIPT} ${_LDFLAGS} \
+SYSTEM_LD_BASECMD= \
+	${LD} -m ${LD_EMULATION} -Bdynamic -T ${LDSCRIPT} ${_LDFLAGS} \
 	--no-warn-mismatch --warn-common --export-dynamic \
-	--dynamic-linker /red/herring \
-	-o ${.TARGET} -X ${SYSTEM_OBJS} vers.o
+	--dynamic-linker /red/herring -X
+SYSTEM_LD= @${SYSTEM_LD_BASECMD} -o ${.TARGET} ${SYSTEM_OBJS} vers.o
 SYSTEM_LD_TAIL= @${OBJCOPY} --strip-symbol gcc2_compiled. ${.TARGET} ; \
 	${SIZE} ${.TARGET} ; chmod 755 ${.TARGET}
 SYSTEM_DEP+= ${LDSCRIPT}