common: fit: Allow U-Boot images to be booted

common: fit: Allow U-Boot images to be booted

In certain circumstances it comes in handy to be able to boot into a second
U-Boot. But as of now it is not possible to boot a U-Boot binary that is inside
a FIT image, which is problematic for projects that e.g. need to guarantee a
unbroken chain of trust from SOC all the way into the OS, since the FIT signing
mechanism cannot be used.

This patch adds the capability to load such FIT images.

An example .its snippet (utilizing signature verification) might look
like the following:

images {
	firmware@1 {
		description = "2nd stage U-Boot image";
		data = /incbin/("u-boot-dtb.img.gz");
		type = "firmware";
		arch = "arm";
		os = "u-boot";
		compression = "gzip";
		load = <0x8FFFC0>;
		entry = <0x900000>;
		signature@1 {
			algo = "sha256,rsa4096";
			key-name-hint = "key";
		};
	};
};

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Tom Rini <trini@konsulko.com>