arch/powerpc/*/config.mk: make CONFIG_SYS_LDSCRIPT settings work

As we try to get rid of board specific config.mk files we must
provide a way for board specific settings of the LDSCRIPT variable
(path to the linker script) where needed.

We now implement the following hierarchy:

- Highest priority has a "#define CONFIG_SYS_LDCONFIG" in the board
  config file.
- If CONFIG_SYS_LDCONFIG is not set, and the system is booting from
  NAND (CONFIG_NAND_SPL is set), then a board specific linker
  script board/$(BOARDDIR)/u-boot-nand.lds gets used.
- If we are not booting from NAND, we test if a processor specific
  linker script arch/powerpc/cpu/$(CPU)/u-boot.lds exists; if so we
  use that.
- As default, arch/powerpc/config.mk gets used.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: Kumar Gala <kumar.gala@freescale.com>
Cc: Andy Fleming <afleming@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
diff --git a/arch/powerpc/cpu/mpc512x/config.mk b/arch/powerpc/cpu/mpc512x/config.mk
index b29edb1..baf55cc 100644
--- a/arch/powerpc/cpu/mpc512x/config.mk
+++ b/arch/powerpc/cpu/mpc512x/config.mk
@@ -1,5 +1,5 @@
 #
-# (C) Copyright 2007-2009 DENX Software Engineering
+# (C) Copyright 2007-2010 DENX Software Engineering
 #
 # See file CREDITS for list of people who contributed to this
 # project.
@@ -19,11 +19,8 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 # MA 02111-1307 USA
 #
+
 PLATFORM_RELFLAGS += -fPIC -meabi
 
 PLATFORM_CPPFLAGS += -DCONFIG_MPC512X -DCONFIG_E300 \
 			-ffixed-r2 -msoft-float -mcpu=603e
-
-# Use default linker script.
-# A board port can override this setting in board/*/config.mk
-LDSCRIPT := $(SRCTREE)/arch/powerpc/cpu/mpc512x/u-boot.lds