spl/85xx: new SPL support

Update CONFIG_RAMBOOT and CONFIG_NAND_SPL references to accept CONFIG_SPL
and CONFIG_SPL_BUILD, respectively.  CONFIG_NAND_SPL can be removed once
the last mpc85xx nand_spl target is gone.

CONFIG_RAMBOOT will need to remain for other use cases, but it doesn't
seem right to overload it for meaning SPL as well as nand_spl does.  Even
if it's somewhat appropriate for the main u-boot, the SPL itself isn't
(necessarily) ramboot, and we don't have separate configs for SPL and
main u-boot.  It was also inconsistent, as other platforms such as
mpc83xx didn't use CONFIG_RAMBOOT in this way.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>