Unrevert revision 1.12. Revision 1.14 backed this out saying it was

backing out the 1024 sector boot0, but revision 1.12 had nothing to do with
that.  Instead, it documented various compile time options for boot0 and
allowed them to be overridden via make.conf or options on the make
command line.

sys/boot/i386/boot0/Makefile

@@ -8,23 +8,33 @@ BINMODE=	444
 
 M4?=	m4
 
-B0FLAGS=0xf
-B0TICKS=0xb6
+# The default set of flags compiled into boot0.  This enables update (writing
+# the modified boot0 back to disk after running so that the selection made is
+# saved), packet mode (detect and use the BIOS EDD extensions if we try to
+# boot past the 1024 cylinder liimt), and booting from all valid slices.
+BOOT_BOOT0_FLAGS?=	0xf
 
-ORG=	0x600
+# The number of timer ticks to wait for a keypress before assuming the default
+# selection.  Since there are 18.2 ticks per second, the default value of
+# 0xb6 (182d) corresponds to 10 seconds.
+BOOT_BOOT0_TICKS?=	0xb6
+
+# The base address that we the boot0 code to to run it.  Don't change this
+# unless you are glutton for punishment.
+BOOT_BOOT0_ORG?=	0x600
 
 boot0: boot0.o
 .if ${OBJFORMAT} == aout
-	${LD} -N -s -T ${ORG} -o boot0.out boot0.o
+	${LD} -N -s -T ${BOOT_BOOT0_ORG} -o boot0.out boot0.o
 	dd if=boot0.out of=${.TARGET} ibs=32 skip=1
 .else
-	${LD} -N -e start -Ttext ${ORG} -o boot0.out boot0.o
+	${LD} -N -e start -Ttext ${BOOT_BOOT0_ORG} -o boot0.out boot0.o
 	objcopy -S -O binary boot0.out ${.TARGET}
 .endif
 
 boot0.o: boot0.s
-	${AS} ${AFLAGS} --defsym FLAGS=${B0FLAGS} --defsym TICKS=${B0TICKS} \
-		${.IMPSRC} -o ${.TARGET}
+	${AS} ${AFLAGS} --defsym FLAGS=${BOOT_BOOT0_FLAGS} \
+		--defsym TICKS=${BOOT_BOOT0_TICKS} ${.IMPSRC} -o ${.TARGET}
 
 CLEANFILES+= boot0.out boot0.o