Fix GCC format-security errors and convert sprintfs.

With format-security errors turned on, GCC picks up the use of sprintf with
a format parameter not being a string literal.

Simple uses of sprintf are also converted to use strcpy.

Signed-off-by: Ben Whitten <ben.whitten@gmail.com>
Acked-by: Wolfgang Denk <wd@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>