add initial README file

Signed-off-by: Yanfeng Liu <[email protected]>

README.md

@@ -0,0 +1,55 @@
+# Introduction
+
+This builds Linux with rv64ilp32 for trying on K230 devices.
+
+# Preparations
+
+Please prepare toolchain, source tree and host packages before building.
+
+## Toolchain preparation
+
+Please get the [rv64ilp32 toolchain](https://github.com/ruyisdk/riscv-gnu-toolchain-rv64ilp32/releases) and unpack it in a folder, let's assume environment variable `RV64ILP32_TOOLCHAIN_HOME` points to that folder:
+
+```
+$ export RV64ILP32_TOOLCHAIN_HOME=<toolchain-install-root>
+$ ls -F $RV64ILP32_TOOLCHAIN_HOME
+riscv/
+```
+
+Please note that the `riscv64-unknown-linux-gnu-` toolchain is needed to build U-Boot, it can be from the [mainline toolchain](https://github.com/riscv-collab/riscv-gnu-toolchain/releases/download/2023.10.18/riscv64-glibc-ubuntu-22.04-gcc-nightly-2023.10.18-nightly.tar.gz). Let's assume this toolchain is available at folder specified by `MAINLINE` environment variable.
+
+## Source tree preparation
+
+Before building, please run the following commands to fetch the submodule sources:
+
+```
+$ git submodule update --init --recursive
+```
+
+## Host packages
+
+The following host packages are also needed on Ubuntu:
+
+```
+$ sudo apt install libssl-dev mtools python3.10-venv
+```
+
+The [genimage tool](https://github.com/pengutronix/genimage/releases) is also needed. It can be build with the host C compiler like below:
+
+```
+$ cd genimage-17
+$ ./configure
+$ make
+$ sudo make install
+$ genimage -h
+```
+
+# Build
+
+Run `build.sh` in top folder to see the list of targets:
+
+```
+PATH=$MAINLINE/bin:$PATH ./build.sh
+```
+
+The final image will be available in `output/` folder, which can be used on CanMV-K230 device. Read `build.sh` for more usage tips.