Blackfin: fix silent crash when no uClinux MTD filesystem exists

Since we need to relocate the attached filesystem with the uClinux MTD map
(to handle some anomalies), we need to know its real filesize.  If we boot
a kernel without a filesystem actually attached, we end up blindly reading
and copying garbage (since there is no magic value to detect validity).
Often times this results in an early crash and no output.  So add a few
basic sanity checks before operating on things to catch the majority of
cases.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>