Solution was to launch the application using shell and a script:
::sysinit:/bin/hostname -F /etc/hostname
::sysinit:/etc/init.d/rcS
#Start my "home-made" application
::sysinit:/bin/sh -c /root/StartAppl.sh
#Put a getty on the serial port
console::respawn:/sbin/getty -L console 0 vt100 # GENERIC_SERIAL
tty1::respawn:/sbin/getty -L tty1 0 vt100 # HDMI console
The StartAppl.sh is a script calling the "appl". Not sure why I have to add a script calling "appl" instead of just calling "appl" directly...