config: Add a default CONFIG_SYS_PROMPT

The definitions for CONFIG_SYS_PROMPT are varied with little reason other
than to display the board name. Over half the definitions are "==> ", so
make this the default. The rest of the boards remain unchanged to avoid
breaking any external scripts expecting a certain prompt.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>