uboot-phytec

mkenvimage: fix usage message

Don't use argv[0] for usage() because it may or may not be clobbered
by the previous call to basename().  Use "prg" instead as it is done
in the rest of the code.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>