tools: add script: mkimage_signed.sh for sign u-boot binary

tools: add script: mkimage_signed.sh for sign u-boot binary

This script appends given binary image by signature header.
Script takes two arguments:
- @arg1: binary name
- @arg2: config name

Script usage:
./mkimage_signed.sh binary.img config_name
e.g.:
./mkimage_signed.sh u-boot-dtb.bin trats2_config

Sign header is stored on last 512 bytes of input binary.
Maximum imput binary size is 1MB - 512 Bytes = 1048064 Bytes.

The maximum size is limited by first bootloader (s-boot).
Now it is configured to load exactly 2048 blocks (1MB).
Current u-boot-dtb.bin size for trats2 is less than 600 KBytes,
so the size limit should not be an issue.

Change-Id: Ia7472be693df8d4135312971d963d27f2ea20f0f
Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>