[dhcp] Fix definitions for x86_64 and EFI BC client architectures

There has been a longstanding disagreement between RFC4578 and the
IANA "Processor Architecture Types" registry.  RFC4578 section 2.1
defines type 7 as "EFI BC" and type 9 as "EFI x86-64"; the IANA
registry quotes RFC4578 as its source but has these values erroneously
swapped.  The EDK2 codebase uses the IANA values.

As of March 2016, RFC4578 has been modified by an errata to match the
values as recorded in the IANA registry.

Fix our definitions to match the consensus values.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/dhcp.h

 	DHCP_CLIENT_ARCHITECTURE_LC = 0x0005,
 	/** EFI IA32 */
 	DHCP_CLIENT_ARCHITECTURE_IA32 = 0x0006,
-	/** EFI BC */
-	DHCP_CLIENT_ARCHITECTURE_EFI = 0x0007,
+	/** EFI x86-64 */
+	DHCP_CLIENT_ARCHITECTURE_X86_64 = 0x0007,
 	/** EFI Xscale */
 	DHCP_CLIENT_ARCHITECTURE_XSCALE = 0x0008,
-	/** EFI x86-64 */
-	DHCP_CLIENT_ARCHITECTURE_X86_64 = 0x0009,
+	/** EFI BC */
+	DHCP_CLIENT_ARCHITECTURE_EFI = 0x0009,
 	/** EFI 32-bit ARM */
 	DHCP_CLIENT_ARCHITECTURE_ARM32 = 0x000a,
 	/** EFI 64-bit ARM */