ubi-utils: various fixes

ubi-utils: various fixes

gcc-4.3.x introduces a number of new warnings (and we use -Werror) for
things like not checking scanf return values and not using explicit
formatting.  Finally, it caught that we were overflowing our own buffer
in unubi.c, so increate the variable size as we want to declare and
clear PATH_MAX + 1, then use only PATH_MAX of the variable.

Signed-off-by: Tom Rini <trini@embeddedalley.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>