da830: modify the U-Boot prompt string

Modify U-boot promt string from 'DA830-evm >' to 'U-Boot >' as
there are many variants of da830 based boards which have diffrent
names such as L137, AM1707 etc.

Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Nagabhushana Netagunte <nagabhushana.netagunte@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>