[build] Pad .rom, .dsk, and .hd images to 512-byte boundaries

QEMU will silently round down a disk or ROM image file to the nearest
512 bytes.  Fix by always padding .rom, .dsk and .hd images to the
nearest 512-byte boundary.

Originally-fixed-by: Stefan Hajnoczi <stefanha@gmail.com>

src/arch/i386/Makefile

@@ -114,13 +114,6 @@ NON_AUTO_MEDIA	+= fd0
 	$(Q)dd if=$< bs=512 conv=sync of=/dev/fd0
 	$(Q)sync
 
-# rule to create padded disk images
-NON_AUTO_MEDIA	+= pdsk
-%pdsk : %dsk
-	$(QM)$(ECHO) "  [DSKPAD] $@"
-	$(Q)cp $< $@
-	$(Q)$(PERL) ./util/dskpad.pl $@
-
 # Add NON_AUTO_MEDIA to the media list, so that they show up in the
 # output of "make"
 #

src/arch/i386/Makefile.pcbios

@@ -27,6 +27,12 @@ MEDIA		+= raw
 MEDIA		+= com
 MEDIA		+= exe
 
+# Padding rules
+#
+PAD_rom		= $(PADIMG) --blksize=512 --byte=0xff $@
+PAD_dsk		= $(PADIMG) --blksize=512 $@
+PAD_hd		= $(PADIMG) --blksize=512 $@
+
 # rule to make a non-emulation ISO boot image
 NON_AUTO_MEDIA	+= iso
 %iso:	%lkrn util/geniso