[build] Avoid deleting config header files if build is interrupted

With extremely unlucky timing, it is possible to interrupt a build and
cause make to delete config/named.h (and possibly any local
configuration headers).

Mark config/named.h and all local configuration headers as .PRECIOUS
to prevent make from ever deleting them.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[prefix] Halt system without burning CPU if we cannot access the payload

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[prefix] Report both %esi and %ecx when opening payload fails

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[util] Use PCI length field to obtain length of individual images

Option::ROM currently uses the initialisation length field (single
byte at offset 0x02) to determine the length of a ROM image within a
multi-image ROM file.  For PCI ROM images with a code type other than
0, the initialisation length field may not be present.

Fix by using the PCI header's image length field instead.  Note that
this does not prevent us from correctly handling ISA ROMs, since ISA
ROMs do not support multiple images within a single ROM BAR anyway.

Inspired-by: Swift Geek <swiftgeek@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[mromprefix] Use PCI length field to obtain length of individual images

mromprefix.S currently uses the initialisation length field (single
byte at offset 0x02) to determine the length of a ROM image within a
multi-image ROM BAR.  For PCI ROM images with a code type other than
0, the initialisation length field may not be present.

Fix by using the PCI header's image length field instead.

Inspired-by: Swift Geek <swiftgeek@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[mromprefix] Allow for .mrom images larger than 128kB

The .mrom payload has a code type of 0xff and so the initialisation
length field (single byte at offset 0x02) does not need to be
present.  Use only the PCI header's image length field, which allows
the .mrom payload to be up to 32MB in size.

Inspired-by: Swift Geek <swiftgeek@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>