env: Move TOTAL_MALLOC_LEN to environment.h

This declaration is only used in three files. Although it relates to
malloc() it is actually only used during malloc() init. It uses CONFIG
options including CONFIG_ENV_ADDR which are defined only in environment.h
so this header must be included anyway, for TOTAL_MALLOC_LEN to be
correct.

Nove it to environment.h to simplify the common file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>