Fixed the shell arithmetic to work with 4.x /bin/sh.

Tested by:	make release of HEAD on 4.10-STABLE

release/Makefile

@@ -1086,7 +1086,7 @@ makeFloppySet:
 	    ${RD}/floppyset/${FLOPPYBASE} ${FLPSPLITSIZE} "${FLOPPYDESC}"
 	( splitfile=${SPLITDIR}/`basename ${SPLITFILE}`.split ; \
 	lines=`cat $${splitfile} | wc -l`; \
-	lines=$$((lines - 1)) ; \
+	lines=$$(($$lines - 1)) ; \
 	for line in `jot $$lines`; do \
 		file=`head -n $$(($${line} + 1)) $${splitfile} | tail -1 | cut -f 1 -d ' '` ; \
 		sh -e ${DOFS_SH} ${RD}/floppies/${FLOPPYBASE}$${line}.flp \

release/scripts/split-file.sh

@@ -35,5 +35,5 @@ echo `basename ${bootchunk}` "\"Boot floppy\"" > ${DEST}/${prefix}.split
 i=1
 for file in ${files}; do
 	echo `basename ${file}` "\"${DESCR} floppy ${i}\"" >> ${DEST}/${prefix}.split
-	i=$((i + 1))
+	i=$(($i + 1))
 done