arm: Correct build error introduced by getenv_ulong() patch

Commit dc8bbea0 removed a local variable that is used in most ARM boards.

Since we want to avoid an 'unused variable' warning with later compilers,
and the #ifdef logic of whether this variable is required is bit painful,
this declares the variable local to the block of code that needs it.

Signed-off-by: Simon Glass <sjg@chromium.org>