powerpc: convert makefiles to Kbuild style

Note:
arch/powerpc/cpu/mpc8260/Makefile is originally like follows:

    ---<snip>---
    START   = start.o kgdb.o
    COBJS   = traps.o serial_smc.o serial_scc.o cpu.o cpu_init.o speed.o \
    ---<snip>---
    COBJS-$(CONFIG_ETHER_ON_SCC) = ether_scc.o
    ---<snip>---
    $(LIB): $(OBJS)
            $(call cmd_link_o_target, $(OBJS) $(obj)kgdb.o)

The link rule `$(call cmd_link_o_target, $(OBJS) $(obj)kgdb.o)'
is weird.
kbdg.o is not included in $(OBJS) but linked into $(LIB)
and $(LIB) is not dependent on kgdb.o.
(Broken dependency tracking)

So,
    START   = start.o kgdb.o
shoud have been
    START   = start.o
    SOBJS   = kgdb.o

That is why this commit adds kgdb.o to obj-y, not to extra-y.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
diff --git a/arch/powerpc/cpu/ppc4xx/Makefile b/arch/powerpc/cpu/ppc4xx/Makefile
index d38b4aa..14b520f 100644
--- a/arch/powerpc/cpu/ppc4xx/Makefile
+++ b/arch/powerpc/cpu/ppc4xx/Makefile
@@ -5,71 +5,49 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
+extra-y	:= resetvec.o
+extra-y	+= start.o
 
-LIB	= $(obj)lib$(CPU).o
+obj-y	:= cache.o
+obj-y	+= dcr.o
+obj-y	+= kgdb.o
 
-START	:= resetvec.o
-START	+= start.o
-
-SOBJS	:= cache.o
-SOBJS	+= dcr.o
-SOBJS	+= kgdb.o
-
-COBJS	:= 40x_spd_sdram.o
+obj-y	+= 40x_spd_sdram.o
 
 ifndef CONFIG_NAND_SPL
 ifndef CONFIG_NAND_U_BOOT
-COBJS	+= 44x_spd_ddr.o
+obj-y	+= 44x_spd_ddr.o
 endif
 endif
-COBJS-$(CONFIG_SDRAM_PPC4xx_IBM_DDR2) += 44x_spd_ddr2.o
-COBJS-$(CONFIG_PPC4xx_DDR_AUTOCALIBRATION) += 4xx_ibm_ddr2_autocalib.o
-COBJS	+= 4xx_pci.o
-COBJS	+= 4xx_pcie.o
-COBJS	+= bedbug_405.o
-COBJS-$(CONFIG_CMD_CHIP_CONFIG)	+= cmd_chip_config.o
-COBJS	+= cpu.o
-COBJS	+= cpu_init.o
-COBJS	+= denali_data_eye.o
-COBJS	+= denali_spd_ddr2.o
-COBJS	+= ecc.o
-COBJS-$(CONFIG_CMD_ECCTEST) += cmd_ecctest.o
-COBJS	+= fdt.o
-COBJS	+= interrupts.o
-COBJS-$(CONFIG_CMD_REGINFO) += reginfo.o
-COBJS	+= sdram.o
-COBJS	+= speed.o
-COBJS	+= tlb.o
-COBJS	+= traps.o
-COBJS	+= usb.o
-COBJS	+= usb_ohci.o
-COBJS-$(CONFIG_XILINX_440) += xilinx_irq.o
+obj-$(CONFIG_SDRAM_PPC4xx_IBM_DDR2) += 44x_spd_ddr2.o
+obj-$(CONFIG_PPC4xx_DDR_AUTOCALIBRATION) += 4xx_ibm_ddr2_autocalib.o
+obj-y	+= 4xx_pci.o
+obj-y	+= 4xx_pcie.o
+obj-y	+= bedbug_405.o
+obj-$(CONFIG_CMD_CHIP_CONFIG)	+= cmd_chip_config.o
+obj-y	+= cpu.o
+obj-y	+= cpu_init.o
+obj-y	+= denali_data_eye.o
+obj-y	+= denali_spd_ddr2.o
+obj-y	+= ecc.o
+obj-$(CONFIG_CMD_ECCTEST) += cmd_ecctest.o
+obj-y	+= fdt.o
+obj-y	+= interrupts.o
+obj-$(CONFIG_CMD_REGINFO) += reginfo.o
+obj-y	+= sdram.o
+obj-y	+= speed.o
+obj-y	+= tlb.o
+obj-y	+= traps.o
+obj-y	+= usb.o
+obj-y	+= usb_ohci.o
+obj-$(CONFIG_XILINX_440) += xilinx_irq.o
 ifndef CONFIG_XILINX_440
-COBJS	+= 4xx_uart.o
-COBJS	+= gpio.o
-COBJS	+= miiphy.o
-COBJS	+= uic.o
+obj-y	+= 4xx_uart.o
+obj-y	+= gpio.o
+obj-y	+= miiphy.o
+obj-y	+= uic.o
 endif
 
 ifdef CONFIG_SPL_BUILD
-COBJS-y += spl_boot.o
+obj-y += spl_boot.o
 endif
-
-SRCS	:= $(START:.o=.S) $(SOBJS:.o=.S) $(COBJS:.o=.c)
-OBJS	:= $(addprefix $(obj),$(SOBJS) $(COBJS) $(COBJS-y))
-START	:= $(addprefix $(obj),$(START))
-
-all:	$(obj).depend $(START) $(LIB)
-
-$(LIB):	$(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################