Generic SPL framework
  =====================
  
  Overview
  --------
  
  To unify all existing implementations for a secondary program loader (SPL)
  and to allow simply adding of new implementations this generic SPL framework
  has been created. With this framework almost all source files for a board
  can be reused. No code duplication or symlinking is necessary anymore.
  
  
  How it works
  ------------
  
  There is a new directory TOPDIR/spl which contains only a Makefile.
  The object files are built separately for SPL and placed in this directory.
  The final binaries which are generated are u-boot-spl, u-boot-spl.bin and
  u-boot-spl.map.
  
  During the SPL build a variable named CONFIG_SPL_BUILD is exported
  in the make environment and also appended to CPPFLAGS with -DCONFIG_SPL_BUILD.
  Source files can therefore be compiled for SPL with different settings.
  ARM-based boards have previously used the option CONFIG_PRELOADER for it.
  
  For example:
  
  ifeq ($(CONFIG_SPL_BUILD),y)
  COBJS-y += board_spl.o
  else
  COBJS-y += board.o
  endif
  
  COBJS-$(CONFIG_SPL_BUILD) += foo.o
  
  #ifdef CONFIG_SPL_BUILD

  	foo();

  #endif
  
  
  The building of SPL images can be with:
  
  #define CONFIG_SPL

  Because SPL images normally have a different text base, one has to be
  configured by defining CONFIG_SPL_TEXT_BASE. The linker script has to be

  defined with CONFIG_SPL_LDSCRIPT.
  
  To support generic U-Boot libraries and drivers in the SPL binary one can
  optionally define CONFIG_SPL_XXX_SUPPORT. Currently following options
  are supported:
  
  CONFIG_SPL_LIBCOMMON_SUPPORT (common/libcommon.o)
  CONFIG_SPL_LIBDISK_SUPPORT (disk/libdisk.o)
  CONFIG_SPL_I2C_SUPPORT (drivers/i2c/libi2c.o)
  CONFIG_SPL_GPIO_SUPPORT (drivers/gpio/libgpio.o)
  CONFIG_SPL_MMC_SUPPORT (drivers/mmc/libmmc.o)
  CONFIG_SPL_SERIAL_SUPPORT (drivers/serial/libserial.o)
  CONFIG_SPL_SPI_FLASH_SUPPORT (drivers/mtd/spi/libspi_flash.o)
  CONFIG_SPL_SPI_SUPPORT (drivers/spi/libspi.o)
  CONFIG_SPL_FAT_SUPPORT (fs/fat/libfat.o)
  CONFIG_SPL_LIBGENERIC_SUPPORT (lib/libgeneric.o)

  CONFIG_SPL_POWER_SUPPORT (drivers/power/libpower.o)

  CONFIG_SPL_NAND_SUPPORT (drivers/mtd/nand/libnand.o)

  CONFIG_SPL_DRIVERS_MISC_SUPPORT (drivers/misc)

  CONFIG_SPL_DMA_SUPPORT (drivers/dma/libdma.o)

  CONFIG_SPL_POST_MEM_SUPPORT (post/drivers/memory.o)

  CONFIG_SPL_NAND_LOAD (drivers/mtd/nand/nand_spl_load.o)

  CONFIG_SPL_SPI_LOAD (drivers/mtd/spi/spi_spl_load.o)

  CONFIG_SPL_RAM_DEVICE (common/spl/spl.c)

  CONFIG_SPL_WATCHDOG_SUPPORT (drivers/watchdog/libwatchdog.o)

  
  Normally CPU is assumed to be the same between the SPL and normal
  u-boot build.  However it is possible to specify a different CPU for
  the SPL build for cases where the SPL is expected to run on a
  different CPU model from the main u-boot.  This is done by specifying
  an SPL CPU in boards.cfg as follows:
  
  	normal_cpu:spl_cpu

  This case CPU will be set to "normal_cpu" during the main u-boot

  build and "spl_cpu" during the SPL build.

  
  
  Debugging
  ---------
  
  When building SPL with DEBUG set you may also need to set CONFIG_PANIC_HANG
  as in most cases do_reset is not defined within SPL.

  
  
  Estimating stack usage
  ----------------------
  
  With gcc 4.6 (and later) and the use of GNU cflow it is possible to estimate
  stack usage at various points in run sequence of SPL.  The -fstack-usage option
  to gcc will produce '.su' files (such as arch/arm/cpu/armv7/syslib.su) that
  will give stack usage information and cflow can construct program flow.
  
  Must have gcc 4.6 or later, which supports -fstack-usage
  
  1) Build normally
  2) Perform the following shell command to generate a list of C files used in
  SPL:
  $ find spl -name '*.su' | sed -e 's:^spl/::' -e 's:[.]su$:.c:' > used-spl.list
  3) Execute cflow:
  $ cflow --main=board_init_r `cat used-spl.list` 2>&1 | $PAGER
  
  cflow will spit out a number of warnings as it does not parse
  the config files and picks functions based on #ifdef.  Parsing the '.i'
  files instead introduces another set of headaches.  These warnings are
  not usually important to understanding the flow, however.