env: make file-scope env_ptr variables static

env: make file-scope env_ptr variables static

The combination ENV_IS_IN_NVRAM=y, ENV_IS_IN_REMOTE=y fails to build:

env/remote.o:/mnt/ext4/devel/u-boot/env/remote.c:17: multiple definition of `env_ptr'
env/nvram.o:/mnt/ext4/devel/u-boot/env/nvram.c:41: first defined here

It's not necessarily a meaningful combination, but for build-testing
it's nice to be able to enable most ENV_IS_IN_* at the same time, and
since these env_ptr are not declared anywhere, they really have no
reason to have external linkage.

nand.c and flash.c similarly already define file-scope static env_ptr
variables.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>