[romprefix] Avoid unaligned accesses within ROM headers

Ensure that all headers (PCI, UNDI, PnP, iPXE) are aligned to at least
four bytes, so that all accesses to header fields will be correctly
aligned even when reading directly from the expansion ROM BAR.

Reported-by: Peter von Konigsmark <peter@exablaze.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/prefix/mromprefix.S

 	.word	0
 	.size	mromheader, . - mromheader
 
+	.align	4
 mpciheader:
 	.ascii	"PCIR"			/* Signature */
 	.word	pci_vendor_id		/* Vendor identification */

src/arch/x86/prefix/romprefix.S

 	.previous
 
 .ifeqs	BUSTYPE, "PCIR"
+	.align	4
 pciheader:
 	.ascii	"PCIR"			/* Signature */
 	.word	pci_vendor_id		/* Vendor identification */ 
 
 	.globl	undiheader	
 	.weak	undiloader
+	.align	4
 undiheader:
 	.ascii	"UNDI"			/* Signature */
 	.byte	undiheader_len		/* Length of structure */
 	.equ undiheader_len, . - undiheader
 	.size undiheader, . - undiheader
 
+	.align	4
 ipxeheader:
 	.ascii	"iPXE"			/* Signature */
 	.byte	ipxeheader_len		/* Length of structure */