[shell] define FINSH_PROMPT as finsh_get_prompt when FINSH_USING_MSH enabled

When MSH enabled, the prompt will change at the two shells. So we should
use dynamic prompt when FINSH_USING_MSH defined.
Merge pull request #257 from grissiom/fix-shell

[shell] define FINSH_PROMPT as finsh_get_prompt when FINSH_USING_MSH enabled