pxe: make the first label the implicit default

If no default label is specified, but a situation arises where the
default label should be used, treat the first label specified as the
default label.

Signed-off-by: Jason Hobbs <jason.hobbs@calxeda.com>
Cc: Ricardo Salveti de Araujo <ricardo.salveti@linaro.org>
Cc: Wolfgang Denk <wd@denx.de>