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>