Parameterize out 7680 (15 * 512) as BOOT2SIZE, similar to sys/boot/i386/zfsboot/...

This is being done to make it easier to change in the future--this action might be
needed sooner rather than later because of gcc 6.3.0 bailing, stating that there
is negative free space left (deficit) in the boot2 bootloader.

MFC after:	2 months
Sponsored by:	Dell EMC Isilon

sys/boot/i386/boot2/Makefile

@@ -66,10 +66,12 @@ boot1.out: boot1.o
 CLEANFILES+=	boot2 boot2.ld boot2.ldr boot2.bin boot2.out boot2.o \
 		boot2.h sio.o
 
+BOOT2SIZE=	7680
+
 boot2: boot2.ld
-	@set -- `ls -l ${.ALLSRC}`; x=$$((7680-$$5)); \
+	@set -- `ls -l ${.ALLSRC}`; x=$$((${BOOT2SIZE}-$$5)); \
 	    echo "$$x bytes available"; test $$x -ge 0
-	${DD} if=${.ALLSRC} of=${.TARGET} obs=7680 conv=osync
+	${DD} if=${.ALLSRC} of=${.TARGET} obs=${BOOT2SIZE} conv=osync
 
 boot2.ld: boot2.ldr boot2.bin ${BTXKERN}
 	btxld -v -E ${ORG2} -f bin -b ${BTXKERN} -l boot2.ldr \