notes:
- this repo depends on github.com/nju-mips/buildroot
- before building rootfs, buildroot needs to be downloaded
- buildroot provides uclibc-gcc, which is used to compile busybox
- busybox is automatically downloaded from git://git.busybox.net/busybox, branch 1_29_stable
- busybox's configure file is located at apps/busybox/config
- halt is similar to poweroff, it terminates the emulator
- it write value 0 to nemu trap address 0xb0000000