arm-bsp/fvp-rd-kronos: Fix setting MMC_IMAGE for arm-systemready-linux-distros

After 2800a26c, the ros.board.mmc.p_mmc_file FVP parameter is wrongly set for Distros
Installation because the ARM_SYSTEMREADY_LINUX_DISTRO_ISO_IMAGE is not set globally.

To fix this, set the MMC_IMAGE for the arm-systemready-linux-distros-*.bb recipes.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>