ppc: fix building arch/ppc/boot/lib/ with make O=

arch/ppc/boot/lib/ reuses zlib_inflate from lib/zlib_inflate but kbuild
does not support two different places utilising the same .o file. It results
in recompile for each build because options to compiler changes etc.
So the trick used here is to make a copy of the required .c files.
Acked-by: Tom Rini <trini@kernel.crashing.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

arch/ppc/boot/lib/Makefile

@@ -2,9 +2,22 @@
 # Makefile for some libs needed by zImage.
 #
 
-CFLAGS_kbd.o	+= -Idrivers/char
+CFLAGS_kbd.o	:= -Idrivers/char
+CFLAGS_vreset.o := -I$(srctree)/arch/ppc/boot/include
 
-lib-y := $(addprefix ../../../../lib/zlib_inflate/, \
-           infblock.o infcodes.o inffast.o inflate.o inftrees.o infutil.o)
-lib-y += div64.o
+zlib  := infblock.c infcodes.c inffast.c inflate.c inftrees.c infutil.c
+	 
+lib-y += $(zlib:.c=.o) div64.o
 lib-$(CONFIG_VGA_CONSOLE) += vreset.o kbd.o
+
+
+# zlib files needs header from their original place
+EXTRA_CFLAGS += -Ilib/zlib_inflate
+
+quiet_cmd_copy_zlib = COPY    $@
+      cmd_copy_zlib = cat $< > $@
+
+$(addprefix $(obj)/,$(zlib)): $(obj)/%: $(srctree)/lib/zlib_inflate/%
+	$(call cmd,copy_zlib)
+
+clean-files := $(zlib)