Added more shell commands for SDRAM tests.

Separated test commands out into "write" and "check". Added "erase" to set SDRAM to 0. "selfrefresh" enters SDRAM self-refresh mode, you can then hold the reset button down for a while without losing SDRAM contents (note this might need a pulldown on SDRAM clock enable to be reliable, not sure yet). If you do this when NOT in self-refresh SDRAM contents should be corrupt after about 10 seconds. "normal" returns to normal mode from self-refresh.