vf610twr: Add default environment in line with other Freescale boards

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
vf610twr: Remove SoC name from U-Boot prompt

We've been dropping SoC name from U-Boot prompt as it increase
complexity for automatic testing and makes line longer for no good
reason.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>