odroid: Add boot script (boot.scr) support

Add boot script (boot.scr) support. If no boot script are
found, it boots as usual.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Tested-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>