u-boot: use spl/u-boot-spl.bin as default spl file

Ever since u-boot introduced the generic spl support in u-boot 2012.10, the
default spl output file has been spl/u-boot-spl.bin and not u-boot-spl.bin,
so use that as a more sane default value.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>