env: Change env_match() to static and remove from header

env: Change env_match() to static and remove from header

This function was used by other parts of U-Boot in the past when
environment was read from underlying device one character at a time.

This is not the case anymore.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>