env: restore old env_get_char() behaviour

With multiple environments, the 'get_char' callback for env
drivers does not really make sense any more because it is
only supported by two drivers (eeprom and nvram).

To restore single character loading for these drivers,
override 'env_get_char_spec'.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>

include/environment.h

@@ -217,17 +217,6 @@ struct env_driver {
 	const char *name;
 	enum env_location location;
 
-	/**
-	 * get_char() - Read a character from the environment
-	 *
-	 * This method is optional. If not provided, a default implementation
-	 * will read from gd->env_addr.
-	 *
-	 * @index: Index of character to read (0=first)
-	 * @return character read, or -ve on error
-	 */
-	int (*get_char)(int index);
-
 	/**
 	 * load() - Load the environment from storage
 	 *