env: Fix env_load_location

Commit 7d714a24 ("env: Support multiple environments") added
static variable env_load_location. When saving environmental
variables, this variable is presumed to have the value set before.
In case the value was set before relocation and U-Boot runs from a
NOR flash, this variable wasn't writable. This causes failure when
saving the environment. To save this location, global data must be
used instead.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Maxime Ripard <maxime.ripard@free-electrons.com>