Multiple power supplies in home-made circuit
Continuing on from mkeith here, if you want to switch the linear LED regulating ciruit on and off, I'd use a second N-channel mosfet. The 'ground' end of the constant-current circuit should attach to the drain of the mosfet, while the source of the mosfet should attach to the actual ground. Run the gate to the arduino and hey presto. 

The whole LED regulator circuit with the switch will probably need about 1.2V or greater to work nicely. Add the LED in and you might have anything up to 5.7V, which is too much for the 5v LM2576. I'd make a crude 1.4V reference using a resistor feeding current into a couple of series diodes, with a cap in parallel with the diodes. Attach the ground pin of the LM2576 to the 1.4V reference and it should regulate around 6.4V, which gives you a bit of margin to work with. See how much heat the whole thing produces and heatsink accordingly- you might get away with only a little.