lib: uuid: Do not enable UUID command SPL

The uuid command is only really useful in U-Boot, but it's useless in
SPL. Worse yet, it pulls in various environment manipulation functions
as it call env_set(). Do not compile the command in in SPL.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>