env: Create a location driver for each location

Set up a location driver for each supported environment location. At
present this just points to the global functions and is not used. A
later patch will switch this over to use private functions in each driver.

There are several special cases here in various drivers to handle
peculiarities of certain boards:

1. Some boards define CONFIG_ENV_IS_IN_FAT and CONFIG_SPL_ENV_SUPPORT but
do not actually load the environment in SPL. The env load code was
optimised out before but with the driver, it is not. Therefore a special
case is added to env/fat.c. The correct fix (depending on board testing
might be to disable CONFIG_SPL_ENV_SUPPORT.

2. A similar situations happens with CONFIG_ENV_IS_IN_FLASH. Some boards
do not actually load the environment in SPL, so to reduce code size we
need to drop that code. A similar fix may be possible with these boards,
or it may be possible to adjust the environment CONFIG settings.

Added to the above is that the CONFIG_SPL_ENV_SUPPORT option does not
apply when the environment is in flash.

Obviously the above has been discovered through painful and time-consuming
trial and error. Hopefully board maintainers can take a look and figure
out what is actually needed.

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

env/sata.c

@@ -125,3 +125,11 @@ void env_relocate_spec(void)
 
 	env_import(buf, 1);
 }
+
+U_BOOT_ENV_LOCATION(sata) = {
+	.location	= ENVL_ESATA,
+	.get_char	= env_get_char_spec,
+	.load		= env_relocate_spec,
+	.save		= env_save_ptr(saveenv),
+	.init		= env_init,
+};