Kconfig: clean up skipping low-level initialization

Put all entries for skipping low-level initialization into a sub-menu.
Use different titles for main U-Boot, SPL, TPL.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>