Where is the / or /sbin file system or directory mounted from on Android? Where does the root file system (ie, / not /system) come from on Android? I can see that /system is mounted

cat /proc/mounts

rootfs / rootfs ro,relatime 0 0
  ...
/dev/block/mtdblock8 /system ext4 ro,noatime,nodiratime,barrier=1,data=ordered,noauto_da_alloc 0 0
/dev/block/mtdblock6 /data ext4 rw,nosuid,nodev,noatime,nodiratime,barrier=1,data=ordered,noauto_da_alloc 0 0


I can see that /system and /data are mounted from mtdblockX devices. And that / is mounted on the 'virtual' type 'rootfs'.

There are a bunch of files and directories in / (I'm interested in /sbin) that don't seem to be mounted on a mount point over /. Where do those files come from? (I want to get RW access to /sbin) Is this done with some 'chroot' magic? -- it it from some 'boot' image? (In this specific example I'm using a Minix Neo X5 with Android)
The rootfs is setup by kernel and init,  /sbin is part of initrd.img aka ramdisk.
Initrd.img is part of boot partition, and needs to be unpacked to be edited.

Here is some good info on boot process and editing  boot partition http://www.slideshare.net/chrissimmonds/android-bootslides20
XDA forum HOWTO: Unpack, Edit, and Repack Boot Images

Also, newer devices are requiring boot partition to be signed, and unless bootloader is unlocked, any editing will cause device not to boot.