samples: threads: Rewrite misleading sample

The basic/threads sample is printing a log that looks like this:

Toggle USR0 LED: Counter = 0
Toggle USR1 LED: Counter = 0
Toggle USR0 LED: Counter = 1
Toggle USR0 LED: Counter = 2
Toggle USR0 LED: Counter = 3
Toggle USR0 LED: Counter = 4

From that log you would think that it was logging when leds were
blinking, but actually the led-threads might have crashed, and it
would just continue logging anyway. All it's doing is executing
printk's at roughly the same frequency as the LED's are blinking.

This patch rewrites the sample to use a FIFO so that the printk's only
trigger if the LEDs are actually blinking.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>