tools/mkimage: Remove duplicate line of code

Recent commits 1a99de2c and
6a590c5f both fixed the same bug in the
same manner.  Unfortunately git was "smart" enough to merge both changes
which resulted in some duplicate code.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

Reordered code and comment a bit.

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