Add busybox for riscv32 and mipsel, update README

Signed-off-by: Harry Chen <i@harrychen.xyz>

Makefile

@@ -64,8 +64,8 @@ ifeq ($(arch), $(filter $(arch), x86_64 aarch64))
 	@mv tmp/bin/busybox $(busybox)
 	@rm -rf tmp && rm busybox.tar.xz
 endif
-ifeq ($(arch), riscv64)
+ifeq ($(arch), $(filter $(arch), riscv64 riscv32 mipsel))
-	@wget https://github.com/rcore-os/busybox-riscv-prebuilts/raw/master/busybox-1.30.1-riscv64/busybox -O $(busybox)
+	@wget https://github.com/rcore-os/busybox-prebuilts/raw/master/busybox-1.30.1-${arch}/busybox -O $(busybox)
 endif
 
 busybox: $(busybox)

README.md

@@ -53,15 +53,18 @@ A rootfs is created at `build/$(arch)` and converted to `qcow2`.
 
 |                    | x86_64 | aarch64 | riscv32 | riscv64 | mipsel |
 | ------------------ | ------ | ------- | ------- | ------- | ------ |
-| ucore              | ✅     | ✅      | ✅     | ✅      | ✅    |
+| ucore              | ✅     | ✅      | ✅     | ✅      | ❗     |
 | rust               | ✅     | ✅      | ✅     | ✅      | ✅    |
 | biscuit            | ✅     | ✅      | ✅     | ✅      | ✅    |
-| nginx (linux only) | ✅     | ✅      | ✅     | ✅      | ✅    |
+| nginx (linux only) | ✅     | ✅      | ❗      | ✅      | ❗      |
 | redis (linux only) | ✅     | ✅      | ✅     | ✅      | ✅    |
-| busybox            | ✅     | ✅      | ❌     | ✅      | 🚧    |
+| busybox            | ✅     | ✅      | ✅     | ✅      | ✅    |
 | alpine rootfs      | ✅     | ✅      | ❌     | ❌      | ❌    |
 | iperf3             | ✅     | ❌      | ❌     | ❌      | ❌    |
 
+
+Note: ❗ means workarounds are used so that they may not work properly. ❌ means failure in compiling or not existed on such platform.
+
 ## How to run real world programs
 
 ### How to use Redis