mkimage: Make genimg_print_size() global

Currently it is used by image.c only, but the the function can be
used to support additional mkimage types like for example kwbimage,
so make this function globally visible.

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>

Edited commit message.

Signed-off-by: Wolfgang Denk <wd@denx.de>