preboot: Introduce CONFIG_USE_PREBOOT and migrate CONFIG_PREBOOT

This is the same migration path as commit b6251db8 ("Kconfig:
Introduce USE_BOOTCOMMAND and migrate BOOTCOMMAND").

I also moved the description in README to the Kconfig help.
I ripped off the sentence about 'LWMON' since it is gone already.

I only let my boards migrate, leaving the rest to platform maintainers.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>