Compare mount output using base of dirname (#771)

* Compare mount output using base of dirname

If you're building from a bind-mounted directory, the build will
fail as it will find two entries to unmount, but a single unmount
will remove them both causing an error. Adding a space means that
the mountpoint will only match with a single mount entry ; the
expected path, rather than the pre bind-mount.

* Switch to awk instead of mount, grep, cut pipes

Retry unmount 5 times and give up, letting the user know that they need to resolve the issue manually

---------

Co-authored-by: David Peverley <pev@analogue-micro.com>
Co-authored-by: Serge Schneider <serge@raspberrypi.com>

export-image/05-finalise/01-run.sh

@@ -83,7 +83,7 @@ cp "$ROOTFS_DIR/etc/rpi-issue" "$INFO_FILE"
 	dpkg -l --root "$ROOTFS_DIR"
 } >> "$INFO_FILE"
 
-ROOT_DEV="$(mount | grep "${ROOTFS_DIR} " | cut -f1 -d' ')"
+ROOT_DEV="$(awk "\$2 == \"${ROOTFS_DIR}\" {print \$1}" /etc/mtab)"
 
 unmount "${ROOTFS_DIR}"
 zerofree "${ROOT_DEV}"