tools: fix format string in tools/imx8image.c

tools: fix format string in tools/imx8image.c

on 32bit systems with musl libc compiler emits
warning: format '%lx' expects argument of type 'long unsigned int', but argument 3 has type 'size_t' {aka 'unsigned int'} [-Wformat=]

to fix this use format length modifier 'z' (size_t) instead of 'l'

Signed-off-by: Milan P. Stanić <mps@arvanta.net>
Reviewed-by: Tom Rini <trini@konsulko.com>