ARM: SPL: Clean up spl.c / spl_nand.c slightly

- Remove includes we don't need
- Switch some printf statements to puts
- Convert some printf statements to debug, introduce new puts statements
  - In most cases saying just "No mkimage signature, assuming
    u-boot.bin" or similar is sufficient.  This also means the non-DEBUG
    case doesn't need printf, in the core of SPL.
  - The other case here is that PLAIN_VERSION provided what we wanted
    already, so just use it.

Signed-off-by: Tom Rini <trini@ti.com>