Use coreboot build system for libpayload, too.

This change makes building coreboot related projects more unified.

Change-Id: I0f1181e2fffde1e03675523f7dc9eef3119052c3
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/71
Tested-by: build bot (Jenkins)
Reviewed-by: Uwe Hermann <uwe@hermann-uwe.de>

payloads/libpayload/arch/powerpc/Makefile.inc

@@ -27,7 +27,8 @@
 ## SUCH DAMAGE.
 ##
 
-TARGETS-y += arch/powerpc/head.S.o arch/powerpc/main.o arch/powerpc/sysinfo.o
-TARGETS-y += arch/powerpc/timer.o arch/powerpc/coreboot.o arch/powerpc/util.S.o
-TARGETS-y += arch/powerpc/exec.S.o arch/powerpc/virtual.o
+head.o-y := head.S
+libc-y += main.c sysinfo.c
+libc-y += timer.c coreboot.c util.S.c
+libc-y += exec.S virtual.c