Update rcS to catch SD mount failure

My RedPitaya fails to mount /opt because /dev/mmcblk0p1 isn't ready yet. Add a retry with a short delay to catch this.
Are you sure checking/mounting once after 1s is OK? I do not know what is causing the delay of the SD card detection, but I believe it would be best to do this in a while loop, with multiple smaller sleeps/delays with the associated (longer) timeout. This way, the mouting will finish as soon as the SD card is available and we cover longer SD problems as well.