cmd: bootm: add ELF file support

Some operating systems (e.g. seL4) and embedded applications are ELF
images. It is convenient to use FIT-images to implement trusted boot.
Added "elf" image type for booting using bootm command.

Signed-off-by: Maxim Moskalets <maximmosk4@gmail.com>
diff --git a/boot/image-fit.c b/boot/image-fit.c
index f6464bc..9253f81 100644
--- a/boot/image-fit.c
+++ b/boot/image-fit.c
@@ -2175,7 +2175,8 @@
 		fit_image_check_os(fit, noffset, IH_OS_TEE) ||
 		fit_image_check_os(fit, noffset, IH_OS_OPENRTOS) ||
 		fit_image_check_os(fit, noffset, IH_OS_EFI) ||
-		fit_image_check_os(fit, noffset, IH_OS_VXWORKS);
+		fit_image_check_os(fit, noffset, IH_OS_VXWORKS) ||
+		fit_image_check_os(fit, noffset, IH_OS_ELF);
 
 	/*
 	 * If either of the checks fail, we should report an error, but