doc: add notes on VisionFive 2 boot

doc/visionfive2.txt

@@ -0,0 +1,30 @@
+Booting Yggdrasil OS on Starfive VisionFive 2 RISC-V board:
+
+* TODO: proper format for initrd image
+* TODO: 0x70000000 can be replaced with a builtin var?
+
+Prerequisites:
+
+* OpenSBI + u-boot (you can use the regular debian installation from Starfive)
+* yggdrasil-kernel.bin
+* initrd.img
+
+Steps:
+
+1. Copy yggdrasil-kernel.bin and initrd.img into some directory and start a TFTP server there
+2. Connect to VF2's serial port, ethernet and enter u-boot
+3. Run the following commands:
+
+# Get an IP address
+dhcp
+# [Optional] set some kernel cmdline params
+setenv bootargs "debug.serial-level=info"
+# Load initrd
+tftpboot 0x70000000 <your-ip-address>:initrd.img
+# Load kernel
+tftpboot ${loadaddr} <your-ip-address>:yggdrasil-kernel.bin
+# Load dtb
+load mmc 1:3 ${fdt_addr_r} dtbs/...-starfive/starfive/${fdtfile}
+fdt resize
+# Enter the kernel
+booti ${loadaddr} 0x70000000:<initrd-size> ${fdt_addr_r}