common/spl: handle properly images with bad checksum

common/spl: handle properly images with bad checksum

load_simple_fit() returns -EPERM for the images with broken signatures.
Unfortunately this may conflict with image loaging selection on the base
of boot phase. See commit 873112db9ce68c38984ff25808dde726f8dd5573
("spl: Support selecting images based on phase in simple FIT").

Thus loading of

	configurations {
		uboot {
			description = "u-boot";
			firmware = "atf";
			loadables = "atf", "tee", "uboot";
		};
	};

with damaged "tee" image may finish without errors. This may results in
board bricking.

This patch fixes commit 873112db9ce68c38984ff25808dde726f8dd5573
("spl: Support selecting images based on phase in simple FIT")
by replacing EPERM with EBADSLT places where it should be done.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>