[efi] Add EFI image format and basic runtime environment

We have EFI APIs for CPU I/O, PCI I/O, timers, console I/O, user
access and user memory allocation.

EFI executables are created using the vanilla GNU toolchain, with the
EXE header handcrafted in assembly and relocations generated by a
custom efilink utility.

src/include/gpxe/features.h

@@ -46,6 +46,7 @@
 #define DHCP_EB_FEATURE_PXE		0x21 /**< PXE format */
 #define DHCP_EB_FEATURE_ELF		0x22 /**< ELF format */
 #define DHCP_EB_FEATURE_COMBOOT		0x23 /**< COMBOOT format */
+#define DHCP_EB_FEATURE_EFI		0x24 /**< EFI format */
 
 /** @} */