Kconfig: Migrate BOARD_LATE_INIT to a select

This option should not really be user selectable.  Note that on PowerPC
we currently only need BOARD_LATE_INIT when CHAIN_OF_TRUST is enabled so be
conditional on that.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com> (for UniPhier)