[fnrec] Add function recorder for debugging

The function recorder is a crash and hang debugging tool.  It logs each
function call into a memory buffer while gPXE runs.  After the machine
is reset, and if the contents of memory have not been overwritten, gPXE
will detect the memory buffer and print out its contents.

This allows developers to see a trace of the last functions called
before a crash or hang.  The util/fnrec.sh script can be used to convert
the function addresses back into symbol names.

To build with fnrec:

    make FNREC=1

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>

src/Makefile.housekeeping

@@ -375,6 +375,35 @@ CFLAGS		+= -Werror
 ASFLAGS		+= --fatal-warnings
 endif
 
+# Function trace recorder state in the last build.  This is needed
+# in order to correctly rebuild whenever the function recorder is
+# enabled/disabled.
+#
+FNREC_STATE	:= $(BIN)/.fnrec.state
+ifeq ($(wildcard $(FNREC_STATE)),)
+FNREC_STATE_OLD := <invalid>
+else
+FNREC_STATE_OLD	:= $(shell cat $(FNREC_STATE))
+endif
+ifeq ($(FNREC_STATE_OLD),$(FNREC))
+$(FNREC_STATE) :
+else
+$(FNREC_STATE) : clean
+$(shell $(ECHO) "$(FNREC)" > $(FNREC_STATE))
+endif
+
+VERYCLEANUP	+= $(FNREC_STATE)
+MAKEDEPS	+= $(FNREC_STATE)
+
+ifeq ($(FNREC),1)
+# Enabling -finstrument-functions affects gcc's analysis and leads to spurious
+# warnings about use of uninitialised variables.
+#
+CFLAGS		+= -Wno-uninitialized
+CFLAGS		+= -finstrument-functions
+CFLAGS		+= -finstrument-functions-exclude-file-list=core/fnrec.c
+endif
+
 # compiler.h is needed for our linking and debugging system
 #
 CFLAGS		+= -include compiler.h