"spl_imgsize" was set as decimal variable by "setexpr"
and this causes wrong image size written by "ext4write".
Preset this val with "0x" prefix allow to fix this issue.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>