tools: fix imximage warning

Fix build warning:

Configuring for MPC837XEMDS board...
imximage.c: In function `imximage_parse_cfg_file':
imximage.c:146: warning: passing argument 2 of `getline' from incompatible pointer type
/usr/include/bits/stdio.h:116: note: expected `size_t *' but argument is of type `uint32_t *'

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>