cd boot workaround for bioses that didn't want to boot >4 image sectors.

boot is a normal binary with a.out again. use 'cdbootblock,' a CDBOOT
variant of bootblock, both from bootblock.s, as the first boot image
that then loads boot, exactly like the bootblock loads boot when booting
from harddisk. the sector numbers (2048 byte iso sectors) are patched in
by writeisofs, like installboot does for bootblock. bootblock unchanged.

boot/installboot.c

@@ -495,8 +495,7 @@ void make_bootable(enum howto how, char *device, char *bootblock,
 			boothdr.a_magic[0]= !A_MAGIC0;
 		} else {
 			readblock(addr, buf, block_size);
-			/* Must skip 16 bytes of 'boot' as that contains code. */
-			memcpy(&boothdr, buf + 16, sizeof(struct exec));
+			memcpy(&boothdr, buf, sizeof(struct exec));
 		}
 		bootf= NULL;
 		dummy.process= boothdr;