feat: network boot also support ext4 disk

- Update docs

Author: initdc

README.md

@@ -16,7 +16,7 @@ pkill -f qemu-system-aarch64
 
 ### Prepare
 
-- Ubuntu 26.04
+- Ubuntu 24.04 (GNU coreutils for building OpenWrt) or Ubuntu 26.04 (for qemu 10 RVA23)
 
   ```
   sudo apt install qemu-system-arm make u-boot-tools
@@ -64,7 +64,7 @@ pkill -f qemu-system-aarch64
   ```sh
   fatload virtio 1 ${kernel_addr_r} /boot/Image
   setenv bootargs root=fe00
-  booti ${kernel_addr_r} - ${fdtcontroladdr}
+  booti ${kernel_addr_r} - ${fdt_addr}
   ```
 
   ```sh

network-boot.md

@@ -25,6 +25,8 @@
 
   then `make tftp`
 
+  load `cpio.gz`
+
   ```sh
   # enter u-boot
   dhcp
@@ -33,7 +35,30 @@
   tftp ${ramdisk_addr_r} openwrt-24.10.5-armsr-armv8-rootfs.cpio.gz
 
   setenv bootargs root=/dev/ram0
-  booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdtcontroladdr}
+  booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr}
+  ```
+
+  load `ext4`
+  
+  <!-- ```sh
+  # get file size in KiB
+  du --block-size=KiB openwrt-24.10.5-armsr-armv8-generic-ext4-rootfs.img
+  106496  openwrt-24.10.5-armsr-armv8-generic-ext4-rootfs.img
+
+  # get more 20% size
+  expr 106496 "*" 6 / 5
+  127795
+  ``` -->
+
+  ```sh
+  # enter u-boot
+  dhcp
+
+  tftp ${kernel_addr_r} Image
+  tftp ${ramdisk_addr_r} openwrt-24.10.5-armsr-armv8-generic-ext4-rootfs.img
+
+  setenv bootargs root=/dev/ram0 rw ramdisk_size=0x${filesize}
+  booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr}
   ```
 
   you can compile it as `boot.scr`, then `make tftp-env`