readme: add flash instructions for nand

Author: frank-w

README.md

@@ -31,6 +31,26 @@ kernelfile=bpi-r4_6.5.0-rc1.tar.gz
 ```
 both configs can be used together to not download anything from my github releases.
 
+## how to build nand-image
+
+To use specifc files you can set them in the sourcefiles_bpi-r*.conf.
+If they are not defined, script tries to download the latest files.
+
+if using r4-2g5, r4pro or r4lite board you should set the variant variable.
+
+```
+variant=bpi-r4-lite
+skipubootdownload=1
+fipfile=bpi-r4lite_spim-nand_ubi_fip.bin
+skipkerneldownload=1
+kernelfile=bpi-r4_6.17.0-rc1-r4lite.tar.gz
+skipinitrddownload=1
+initrd=rootfs_arm64.cpio.zst
+```
+finally run build process
+```
+./ubifs.sh bpi-r4
+```
 ## how add packages
 
 add this option in the sourcefiles_board.conf
@@ -41,10 +61,33 @@ userpackages="ethtool iperf3 tcpdump"
 
 ## how to write image
 
+### sd/emmc
+
+flash from linux host
+
 ```sh
 gunzip -c bpi-r3_sdmmc.img.gz | sudo dd bs=1M status=progress conv=notrunc,fsync of=/dev/sdX
 ```
 
+### nand
+
+how to write bl2 and ubinized image to nand in uboot for R4
+
+```sh
+# erase full nand or only bl2 section
+mtd erase spi-nand0
+mtd erase spi-nand0 0x0 0x200000
+# load bl2 and flash it
+fatload usb 0:1 $loadaddr bpi-r4pro_spim-nand_ubi_bl2.img
+mtd write spi-nand0 $loadaddr 0x0 $filesize
+# erase ubi mtd partition (if not done full erase)
+mtd erase spi-nand0 0x200000
+# load and flash ubinized image
+fatload usb 0:1 $loadaddr bpi-r4-pro_nand.img
+mtd write spi-nand0 $loadaddr 0x200000 $filesize
+```
+make sure size of ubi partition in devicetree matches your nand size on first linux boot to correctly resize the ubifs, else EC errors occour when booting from nand.
+
 ## first bootup
 
 ### login