mkenvimage: Fix compiler warning

mkenvimage: Fix compiler warning

Fix the compiler warning

mkenvimage.c: In function ‘main’:
mkenvimage.c:218: warning: format ‘%ld’ expects type ‘long int’, but argument 3 has type ‘unsigned int’
mkenvimage.c:226: warning: format ‘%ld’ expects type ‘long int’, but argument 3 has type ‘unsigned int’

introduced with the commit

mkenvimage: Use mmap() when reading from a regular file
6ee39f8055680654f9cc97b98dcce9588f1ab71e

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>
CC: David Wagner <david.wagner@free-electrons.com>
CC: Anatolij Gustschin <agust@denx.de>
CC: Mike Frysinger <vapier@gentoo.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: David Wagner <deubeuliou@gmail.com>