env: fat: Make the debug messages play a little nicer

Since we have global messages to indicate what's going on, the custom
messages in the environment drivers only make the output less readable.

Make FAT play a little nicer by removing all the extra \n and formatting
that is redundant with the global output.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>