OK here what you want to do.
According the the man page,
RC.BOOT(5) Debian GNU/Linux Manual RC.BOOT(5)
/etc/rc.boot - directory for local or per-package boot scripts.
The /etc/rc.boot directory is obsolete. It has been superseded by the /etc/rcS.d direc_
tory. At boot time, first the /etc/rcS.d directory is scanned and then, for backwards
compatibility, the /etc/rc.boot directory.
The directory /etc/rc.boot is scanned by the program run-parts(8). The scripts found
that have valid filenames for the run-parts program will be run in lexical sort order
of the filenames.