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>
diff --git a/configs/a3y17lte_defconfig b/configs/a3y17lte_defconfig
index b433da3..7ca7ce0 100644
--- a/configs/a3y17lte_defconfig
+++ b/configs/a3y17lte_defconfig
@@ -11,8 +11,11 @@
 CONFIG_FIT=y
 CONFIG_USE_PREBOOT=y
 CONFIG_PREBOOT="echo Read pressed buttons status;KEY_VOLUMEUP=gpa20;KEY_HOME=gpa17;KEY_VOLUMEDOWN=gpa21;KEY_POWER=gpa00;PRESSED=0;RELEASED=1;if gpio input $KEY_VOLUMEUP; then setenv VOLUME_UP $PRESSED; else setenv VOLUME_UP $RELEASED; fi;if gpio input $KEY_VOLUMEDOWN; then setenv VOLUME_DOWN $PRESSED; else setenv VOLUME_DOWN $RELEASED; fi;if gpio input $KEY_HOME; then setenv HOME $PRESSED; else setenv HOME $RELEASED; fi;if gpio input $KEY_POWER; then setenv POWER $PRESSED; else setenv POWER $RELEASED; fi;"
+CONFIG_SAVE_PREV_BL_FDT_ADDR=y
+CONFIG_SAVE_PREV_BL_INITRAMFS_START_ADDR=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_DM_I2C_GPIO=y
+CONFIG_LMB_MAX_REGIONS=64