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.
USB hang workaround. Reduce delay during SDRAM init.

USB hang on software reset seems to be fixed by adding a 1ms delay.
In SDRAM demo, 100ms delay between CKE and first command has been reduced to 1ms, since SDRAM datasheet and 429 RM indicate that 100us is sufficient.
Merge pull request #2 from trepidacious/master

Added more shell commands for SDRAM tests.