[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/Makefile.housekeeping

@@ -693,6 +693,15 @@ $(ZBIN) : util/zbin.c util/nrv2b.c $(MAKEDEPS)
 	$(Q)$(HOST_CC) -O2 -o $@ $<
 CLEANUP += $(ZBIN)
 
+###############################################################################
+#
+# The EFI custom linker
+#
+$(EFILINK) : util/efilink.c $(MAKEDEPS)
+	$(QM)$(ECHO) "  [HOSTCC] $@"
+	$(Q)$(HOST_CC) -O2 -o $@ $< -lbfd
+CLEANUP += $(EFILINK)
+
 ###############################################################################
 #
 # Auto-incrementing build serial number.  Append "bs" to your list of