Convert CONFIG_ENV_IS_IN_MMC/NAND/UBI and NOWHERE to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_MMC
   CONFIG_ENV_IS_IN_NAND
   CONFIG_ENV_IS_IN_UBI
   CONFIG_ENV_IS_NOWHERE

In fact this already exists for sunxi as a 'choice' config. However not
all the choices are available in Kconfig yet so we cannot use that. It
would lead to more than one option being set.

In addition, one purpose of this series is to allow the environment to be
stored in more than one place. So the existing choice is converted to a
normal config allowing each option to be set independently.

There are not many opportunities for Kconfig updates to reduce the size of
this patch. This was tested with

   ./tools/moveconfig.py -i CONFIG_ENV_IS_IN_MMC

And then manual updates.  This is because for CHAIN_OF_TRUST boards they
can only have ENV_IS_NOWHERE set, so we enforce that via Kconfig logic
now.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>