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>

common/board_r.c

@@ -445,6 +445,11 @@ static int initr_env(void)
 		env_set_hex("fdtcontroladdr",
 			    (unsigned long)map_to_sysmem(gd->fdt_blob));
 
+	#if (CONFIG_IS_ENABLED(SAVE_PREV_BL_INITRAMFS_START_ADDR) || \
+						CONFIG_IS_ENABLED(SAVE_PREV_BL_FDT_ADDR))
+		save_prev_bl_data();
+	#endif
+
 	/* Initialize from environment */
 	image_load_addr = env_get_ulong("loadaddr", 16, image_load_addr);