[build] Disable ccache for embedded.o

Embedded image support uses .incbin in inline assembly to include binary
files.  The file dependency is not spotted by ccache when deciding
whether or not to rebuild embedded.o.  This results in builds that
contain an outdated version of the embedded image when ccache is used.

Reported-by: Tim 'Shaggy' Bielawa <tbielawa@jabber.org>
Reported-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>

src/Makefile.housekeeping

 			     \"$(notdir $(word $(i),$(EMBEDDED_FILES)))\" ))
 
 $(BIN)/embedded.o : $(EMBEDDED_FILES) $(EMBEDDED_LIST)
+
+# This file uses .incbin inline assembly to include a binary file.
+# Unfortunately ccache does not detect this dependency and caches builds even
+# when the binary file has changed.
+#
+$(BIN)/embedded.o : override CC := env CCACHE_DISABLE=1 $(CC)
+
 CFLAGS_embedded = -DEMBED_ALL="$(EMBED_ALL)"
 
 # Generate the NIC file from the parsed source files.  The NIC file is