[Android] can't create '/system/bin/busybox': read-only filesystem

You just need to change the read-only option of the mounted filesystem. (FYI it uses yaffs2 filesystem)

In my case,

$ adb shell

$ su

$ mount -o rw,remount /dev/block/mmcblk0p4 /system

application: how to change init.rc

$ mount -o remount,rw rootfs /

and change it!!