ppc4xx: Fix O=buildir builds

This patch fixes the problem to assemble cpu/ppc4xx/start.S
experienced last week where building failed having specified
O=../build.sequoia.

Signed-off-by: Niklaus Giger <niklaus.giger@nestal.com>

cpu/ppc4xx/config.mk

@@ -24,7 +24,7 @@
 PLATFORM_RELFLAGS += -fPIC -ffixed-r14 -meabi -fno-strict-aliasing
 PLATFORM_CPPFLAGS += -DCONFIG_4xx -ffixed-r2 -ffixed-r29 -mstring -msoft-float
 
-cfg=$(shell grep configs $(TOPDIR)/include/config.h | sed 's/.*<\(configs.*\)>/\1/')
+cfg=$(shell grep configs $(OBJTREE)/include/config.h | sed 's/.*<\(configs.*\)>/\1/')
 is440=$(shell grep CONFIG_440 $(TOPDIR)/include/$(cfg))
 
 ifneq (,$(findstring CONFIG_440,$(is440)))