config: exynos: common: Provide env variables to support Image.itb

This change allows using Image.itb image format with Exynos4 devices
(especially trats and trats2).
Such change facilitates automated testing since only one binary needs
to be prepared.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>