ARM1136: Introduce CONFIG_PRELOADER macro.

Currently CONFIG_ONENAND_IPL is used in a number of #ifdef's
in start.S. In preparation for adding support for NAND SPL
the macro CONFIG_PRELOADER is introducted and replaces the
CONFIG_ONENAND_IPL in start.S.

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>