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/cmd/Kconfig b/cmd/Kconfig
index 40ac5a8..68947bf 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -321,6 +321,13 @@
 	help
 	  Support booting UEFI FIT images via the bootm command.
 
+config BOOTM_ELF
+	bool "Support booting ELF images"
+	depends on CMD_BOOTM && LIB_ELF
+	default n
+	help
+	  Support booting ELF images via the bootm command.
+
 config CMD_BOOTZ
 	bool "bootz"
 	help