env: Rework ENV_IS_EMBEDDED and related logic slightly

env: Rework ENV_IS_EMBEDDED and related logic slightly

- Drop CONFIG_BUILD_ENVCRC as this is never set directly but instead
  means ENV_IS_EMBEDDED, so reference that in code and rename the Makefile
  usage to BUILD_ENVCRC.
- Remove extra-$(CONFIG_ENV_IS_EMBEDDED) line as it could never be true,
  and likely why there is an extra- line for CONFIG_ENV_IS_IN_FLASH (the
  only use case today of embedded environments).
- With these slight changes we can then see that using the calculated
  symbol of ENV_IS_EMBEDDED is the right thing to use in any code which
  needs to know this situation and can remove CONFIG_ENV_IS_EMBEDDED
  entirely.

Signed-off-by: Tom Rini <trini@konsulko.com>