pitx-imx8m: update README

- add eMMC boot description.
- add bootloader update package description.
Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>

board/kontron/pitx-imx8m/README

@@ -44,3 +44,40 @@ Boot
 ====
 Insert the uSD card into the upper cage of J12. Be cautious as the lower cage
 is for a uSIM card. Power on the board.
+
+eMMC Boot
+=========
+It is also possible to install the boot image on eMMC boot0 partition. This
+can be done using a system booted from SD card into linux shell. First, the
+flash.bin firmware image has to be copied to the file system of the running
+linux (e.g. /tmp). Then, the image is copied to mmcblk0boot0 partition.
+
+$ dd if=flash.bin of=/dev/mmcblk0boot0 bs=1k seek=33
+
+Before starting boot firmware from eMMC, make sure that BOOT_MODE pins
+SW1[1:2] are set to 0b01. Power on the board.
+
+Bootloader Update
+=================
+
+Bootloader update of the pitx-imx8m uses the distroboot feature. The new
+bootloader is installed on the CPU bootdevice which is determined by SW1[1:2]
+jumper settings.
+
+SW1[1:2]	Boot Device	Update Installation Target
+0b00		SD Card		mmc dev 1
+0b01		eMMC		mmc dev 0 1
+
+Note: The bootloader update package always copies the new bootloader to the
+device the module was booted from. This means, when the module was booted
+from SD card, the update package will install the new bootloader on SD card
+as well. The steps below show how to install a bootloader update on the
+pITX-iMX8M module:
+
+- Extract the bootloader update package as provided by Kontron into the root
+  directory of either USB thumb device, SD card or onboard eMMC.
+- Make sure that no boot.scr or boot.scr.uimg files are installed in the root
+  directory of the update package storage device used.
+- From Bootloader CLI, invoke the appropriate distroboot command, e.g. for
+  USB thumb device:
+  => run bootcmd_usb0