mvebu: image: introduce BOOT_SCRIPT variable

mvebu: image: introduce BOOT_SCRIPT variable

All of U-Boot scripts repeat the same pattern with only Device Tree blob
name changing for respective device. Therefore create generic scripts
which will be altered on demad by image build process, and create
BOOT_SCRIPT variable which can be added to device recipe and will allow
referencing the same script by many device recipes. This will allow to
slim down the ammount of files in buildroot tree and avoid needlessly
incrementing amount of boot scripts if new devices will be added.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>