post/Makefile: fix dependency problem with parallel builds

Parallel builds (using "make -jN") would occasionally fail with error
messages like
	ppc_4xxFP-objdump: string.o: File format not recognized
or
	post/libpost.a(cpu.o): In function `cpu_post_test':
	/home/wd/git/u-boot/work/post/lib_ppc/cpu.c:130: undefined reference to `cpu_post_test_string'
or similar. We now make sure to run the 'postdeps" step before
attempting to build the specific POST libraries.

Signed-off-by: Wolfgang Denk <wd@denx.de>

post/Makefile

@@ -45,7 +45,7 @@ SRCS	:= $(SOBJS:.o=.S) $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(COBJS))
 LIB	:= $(obj)$(LIB)
 
-all:	postdeps $(LIB)
+all:	$(LIB)
 
 postdeps:
 	@for lib in $(SPLIB-y) ; do \
@@ -57,7 +57,7 @@ $(GPLIB): $(obj).depend $(OBJS)
 	$(AR) $(ARFLAGS) $@ $(OBJS)
 
 # specific POST libraries
-$(SPLIB): $(obj).depend
+$(SPLIB): $(obj).depend postdeps
 	$(MAKE) -C $(dir $(subst $(obj),,$@))
 
 # the POST lib archive