mkimage: fit: don't create image with 0700 permissions

mkimage: fit: don't create image with 0700 permissions

commit 7298e422504e ("mkimage: fit: add support to encrypt image with
aes") added a new copyfile() function as part of the FIT image creation
flow. This function as currently written creates the final image with a
mode of 0700 (before umask), differing from the old behavior of 0666.
Since there doesn't seem to be any reason to make the image executable
or non-group, non-other readable, change the mask to 0666 to preserve
the old behavior.

Fixes: 7298e422504e ("mkimage: fit: add support to encrypt image with aes")
Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>