[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/efi/efi_timer.h

@@ -0,0 +1,16 @@
+#ifndef _GPXE_EFI_TIMER_H
+#define _GPXE_EFI_TIMER_H
+
+/** @file
+ *
+ * gPXE timer API for EFI
+ *
+ */
+
+#ifdef TIMER_EFI
+#define TIMER_PREFIX_efi
+#else
+#define TIMER_PREFIX_efi __efi_
+#endif
+
+#endif /* _GPXE_EFI_TIMER_H */