[build] Pad .hd image type to 32 KB

The disk partition prefix code in hdprefix.S reads the gPXE image in
tracks, not individual sectors.  This means it will attempt to read
beyond the end of the image if the .hd image type is not padded to 32
KB.

This issue is affects virtualization software which may execute a .hd or
.usb image file directly - effectively running a machine with a tiny
disk containing just the gPXE image.  Boot will fail when gPXE tries to
read beyond the end of disk.

src/arch/i386/Makefile.pcbios

 #
 PAD_rom		= $(PADIMG) --blksize=512 --byte=0xff $@
 PAD_dsk		= $(PADIMG) --blksize=512 $@
-PAD_hd		= $(PADIMG) --blksize=512 $@
+PAD_hd		= $(PADIMG) --blksize=32768 $@
 
 # rule to make a non-emulation ISO boot image
 NON_AUTO_MEDIA	+= iso