hello_world.c

Try to install manually to get an idea about what is wrong.
The armbian-config tool basically does this when selecting tvheadend for installation:
 
https://github.com/armbian/config/blob/c9a9b494c3069807471bb00c0336566cd186f6d9/debian-software#L615-L635

.config file matching the installed kernel is always in /boot.

The work around [trusted=yes] no longer works. As Igor said, Bionic one is not signed. 
 
If you must, 
 
$ sudo apt update -o Acquire::AllowInsecureRepositories=true -o Acquire::AllowDowngradeToInsecureRepositories=true
 
for the time being while it's being solved (hopefully, it is).
 
 

Well - the LEDs are not statically defined, because you can define their usage by yourself - and the OS can have another default funtion like on a other OS (Orange Pi original Linux <--> armbian)
 
Your could find the available LEDs here:
root@pihole:~# ls -l /sys/class/leds
total 0
lrwxrwxrwx 1 root root 0 Jan  1  1970 green_led -> ../../devices/platform/leds-gpio/leds/green_led
lrwxrwxrwx 1 root root 0 Jan  1  1970 red_led -> ../../devices/platform/leds-gpio/leds/red_led
 
and which function is assigned to the LED you could find out while do a "more" an their trigger-file:
root@pihole:~# more /sys/class/leds/red_led/trigger
none mmc0 mmc1 timer [heartbeat] backlight default-on
 
here you could see I assigned the funtion [heartbeat] to the red LED
 
I did this in the /etc/rc.local with the following command (before the line with "exit 0") :
echo "heartbeat" > /sys/class/leds/red_led/trigger
 
heartbeat is this flashing LED - showing the system is running
 
mmc0 will be like a HDD-LED for your uSD-Card:
echo "mmc0" > /sys/class/leds/green_led/trigger
 
The names of your LEDs may varies.