km/common: introduce $uimage variable

Replace the hardcoded string with a variable. If CONFIG_NAND_ECC_BCH is
set we use a specific name for the uImage (ecc_bch_uImage).

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>