samsung: common: add example boot scripts

This commit adds example scripts of boot.scr:
- bootzimg.cmd - check if dtb exists and boot zImage
- autoboot.cmd - check which image exists: Image.itb, zImage or uImage
                 and optionally load fdt file for u/zImage

The blank spaces are added to improve readability and can be removed
before use mkimage.

Required U-Boot environment variables: $boardname, $fdtfile, $console,
$mmcbootdev, $mmcbootpart, $mmcrootdev, $mmcrootpart, $rootfstype.

Making boot.scr from file.cmd:
 mkimage -C none -A arm -T script -d file.cmd boot.scr

The Odroid XU3 default environment is ready for those boot scripts and
the right script can be loaded by DFU.

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