env: Add CONFIG_SYS_RELOC_GD_ENV_ADDR symbol

Today in initr_reloc_global_data() we use some non-obvious tests to
determine if we need to relocate the env_addr within gd or not.  In
order to facilitate migration of other symbols to Kconfig we need to
introduce a new symbol for this particular use case.

Cc: Wolfgang Denk <wd@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
---
Note, after reading and re-reading the comment as well as looking at
what boards do / do not actually run this one line of code I am of the
opinion that later on we should change to doing this unconditionally.
However, to have a "zero change" Kconfig migration of other symbols we
need to have an option here to avoid some otherwise VERY messy
preprocessor logic tests.