spl: Change debug to printf for "Unsupported boot-device"

We had the problem on an AM33xx platform, that SPL detected an
unsupported boot-device. But since this message is a debug message
it took a bit of time to really know, where the hangup in SPL
resulted from. So let's change this debug message to a printf
and also print the detected boot-device that is not supported.
This makes debugging of such cases much easier.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>
Acked-by: Heiko Schocher <hs@denx.de>