arm: init: save previous bootloader data

When u-boot is used as a chain-loaded bootloader (replacing OS kernel),
previous bootloader leaves data in RAM, that can be reused.

For example, on recent arm linux system, when chainloading u-boot,
there are initramfs and fdt in RAM prepared for OS booting. Initramfs
may be modified to store u-boot's payload, thus providing the ability to
use chainloaded u-boot to boot OS without any storage support.

Two config options added:
- SAVE_PREV_BL_INITRAMFS_START_ADDR
  saves initramfs start address to 'prevbl_initrd_start_addr' environment
  variable
- SAVE_PREV_BL_FDT_ADDR
  saves fdt address to 'prevbl_fdt_addr' environment variable

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Cc: Tom Rini <trini@konsulko.com>

include/init.h

@@ -155,6 +155,19 @@ int arch_setup_bdinfo(void);
  */
 int setup_bdinfo(void);
 
+#if defined(CONFIG_SAVE_PREV_BL_INITRAMFS_START_ADDR) || \
+defined(CONFIG_SAVE_PREV_BL_FDT_ADDR)
+/**
+ * save_prev_bl_data - Save prev bl data in env vars.
+ *
+ * When u-boot is chain-loaded, save previous bootloader data,
+ * like initramfs address to environment variables.
+ *
+ * Return: 0 if ok; -ENODATA on error
+ */
+int save_prev_bl_data(void);
+#endif
+
 /**
  * cpu_secondary_init_r() - CPU-specific secondary initialization
  *