Add script to change kernel boot logo

This is a fix for [746-000045] logo: Add fngpostinstall step to make logo.png appear as kernel logo

A PNG logo can be added to the boot partition during installation via the "--Image" parameter of the fng-install.sh.

During the postinstallation phase, the prepare-logo.sh script generates a logo.dat blob in the boot partition which can be used by the custom logo driver.

During boot, the bootloader reads this blob into the memory. The logo driver will get logo memory address and size from dtb. If this points to a correct data, logo, loaded into memory will be displayed.

The prepare-logo.sh can be also executed later by user to exchange the logo:

• manually copy logo file (png or pnm) to target rootfs
• manually call every script in /etc/fng-postinstall
• reboot

It is expected that customer will provide the boot logo in suitable resolution.