[CLEA-OS] Make IMAGE_CLEA_OS_BOOT_FILES compatible with appeding
A weak assignment of IMAGE_CLEA_OS_BOOT_FILES doesn't allow possible appending. In order to manage the possible configuration for the various bootloaders (U-Boot, Grub, systemd-boot) the EFI_PROVIDER variable is used.
Signed-off-by: Davide Cardillo davide.cardillo@seco.com