Question: Where does debugfs actually reside? On the microSD or on the beaglebone hardware itself?
mount should tell you that:
debugfs on /sys/kernel/debug type debugfs (rw,none)
The kernel provides it, it's not a disk backed file system.
tmpfs (if that's what you meant to ask about) resides where ever you
want it to. Often /tmp is a tmpfs, for example.
https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt
-Andrew