[build] Generate random build identifier

Randomly generate a 32-bit build identifier that can be used to
identify identical iPXE ROMs when multiple such ROMs are present in a
system (e.g. when a multi-function NIC exposes the same iPXE ROM image
via each function's expansion ROM BAR).

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

src/Makefile.housekeeping

 	$(Q)$(RANLIB) $@
 blib : $(BLIB)
 
+# Command to generate build ID.  Must be unique for each $(BIN)/%.tmp,
+# even within the same build run.
+#
+BUILD_ID_CMD	:= perl -e 'printf "0x%08x", int ( rand ( 0xffffffff ) );'
+
 # Build an intermediate object file from the objects required for the
 # specified target.
 #
-$(BIN)/%.tmp : $(BLIB) $(MAKEDEPS) $(LDSCRIPT) 
+$(BIN)/%.tmp : $(BLIB) $(MAKEDEPS) $(LDSCRIPT)
 	$(QM)$(ECHO) "  [LD] $@"
 	$(Q)$(LD) $(LDFLAGS) -T $(LDSCRIPT) $(TGT_LD_FLAGS) $(BLIB) -o $@ \
-		-Map $(BIN)/$*.tmp.map
+		--defsym _build_id=`$(BUILD_ID_CMD)` -Map $(BIN)/$*.tmp.map
 	$(Q)$(OBJDUMP) -ht $@ | $(SORTOBJDUMP) >> $(BIN)/$*.tmp.map
 
 # Keep intermediate object file (useful for debugging)

src/arch/i386/prefix/romprefix.S

 	.long	0
 	.previous
 
+build_id:
+	.long	_build_id		/* Randomly-generated build ID */
+
 pciheader:
 	.ascii	"PCIR"			/* Signature */
 	.word	pci_vendor_id		/* Vendor identification */