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.c b/boot/image.c
index fc774d6..abac254 100644
--- a/boot/image.c
+++ b/boot/image.c
@@ -130,7 +130,10 @@
 	{	IH_OS_OPENRTOS,	"openrtos",	"OpenRTOS",		},
 #endif
 	{	IH_OS_OPENSBI,	"opensbi",	"RISC-V OpenSBI",	},
-	{	IH_OS_EFI,	"efi",		"EFI Firmware" },
+	{	IH_OS_EFI,	"efi",		"EFI Firmware"		},
+#ifdef CONFIG_BOOTM_ELF
+	{	IH_OS_ELF,	"elf",		"ELF Image"		},
+#endif
 
 	{	-1,		"",		"",			},
 };