board: axy17lte: get board usable - add bootcmd and docs

U-boot is intended to replace linux kernel in android boot image(ABL), and
it's FIT payload to replace initramfs file. The boot process is similar to
boot image with linux:
- android bootloader (ABL) unpacks android boot image
- ABL sets `linux,initrd-start property` in chosen node in unpacked FDT
- ABL sets x0 register to FDT address, and passes control to u-boot
- u-boot reads x0 register, and stores it in `prevbl_fdt_addr` env variable
- u-boot reads `linux,initrd-start` property,
and stores it in `prevbl_initrd_start_addr`

In this way, u-boot bootcmd relies on `prevbl_initrd_start_addr` env
variable, and boils down to `bootm $prevbl_initrd_start_addr`.
If more control on boot process is desired, pack a boot script in
FIT image, and put it to default configuration

What done:
- Rearrange defconfig option order
- Add CONFIG_SAVE_PREV_BL_* options
- Doc updates:
  - remove wrong SBOOT memory corruption note, because
  memory is changed during u-boot bringup process,
  not by SBOOT
  - put payload on ramdisk place in abl boot image
  creation step

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>

include/configs/exynos78x0-common.h

@@ -35,6 +35,7 @@
 
 #define CONFIG_SYS_SDRAM_BASE		0x40000000
 #define CONFIG_SYS_INIT_SP_ADDR		(CONFIG_SYS_TEXT_BASE + SZ_2M - GENERATED_GBL_DATA_SIZE)
+#define CONFIG_SYS_BOOTM_LEN	SZ_32M
 /* DRAM Memory Banks */
 #define SDRAM_BANK_SIZE		(256UL << 20UL)	/* 256 MB */
 #define PHYS_SDRAM_1		CONFIG_SYS_SDRAM_BASE
@@ -79,7 +80,10 @@
 #define EXYNOS_FDTFILE_SETTING
 #endif
 
+/* Cannot use bootdelay > 0, because timer is not working */
 #define EXTRA_ENV_SETTINGS \
+	"bootdelay=0\0" \
+	"bootcmd=source $prevbl_initrd_start_addr:bootscript\0"	\
 	EXYNOS_DEVICE_SETTINGS \
 	EXYNOS_FDTFILE_SETTING \
 	MEM_LAYOUT_ENV_SETTINGS