pxe: fix handling of different localboot values

Add support for value of -1 For localboot. A value of -1 means return to
u-boot prompt.

The localboot value is often 0, so we need to distinguish the value from
localboot being selected. A value of greater than or equal to 0 means
attempt local boot command.

If localboot is selected, we don't want to try other entries.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>