image: Add IH_OS_EFI for EFI chain-load boot

image: Add IH_OS_EFI for EFI chain-load boot

Add a new OS type to be used for chain-loading an EFI compatible
firmware or boot loader like GRUB2, possibly in a verified boot
scenario.

Bellow is sample ITS file that generates a FIT image supporting
secure boot. Please note the presence of 'os = "efi";' line, which
identifies the currently introduced OS type:

/ {
    #address-cells = <1>;

    images {
        efi-grub {
            description = "GRUB EFI";
            data = /incbin/("bootarm.efi");
            type = "kernel_noload";
            arch = "arm";
            os = "efi";
            compression = "none";
            load = <0x0>;
            entry = <0x0>;
            hash-1 {
                algo = "sha256";
            };
        };
    };

    configurations {
        default = "config-grub";
        config-grub {
            kernel = "efi-grub";
            signature-1 {
                algo = "sha256,rsa2048";
                sign-images = "kernel";
            };
        };
    };
};

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>