Kconfig: Introduce USE_BOOTCOMMAND and migrate BOOTCOMMAND

We first introduce CONFIG_USE_BOOTCOMMAND, similar to
CONFIG_USE_BOOTARGS.  We then migrate CONFIG_BOOTCOMMAND for most
CONFIG_DISTRO_DEFAULT users.  In some cases platforms have a complex
scheme around this usage, and these have been defered for the moment so
that platform maintainers can work on a migration plan.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>