efi_capsule: Move signature from DTB to .rodata

The capsule signature is now part of our DTB.  This is problematic when a
user is allowed to change/fixup that DTB from U-Boots command line since he
can overwrite the signature as well.
So Instead of adding the key on the DTB, embed it in the u-boot binary it
self as part of it's .rodata.  This assumes that the U-Boot binary we load
is authenticated by a previous boot stage loader.

Reviewed-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Tested-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Tested-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
index 7a469f2..dacc3b5 100644
--- a/lib/efi_loader/Kconfig
+++ b/lib/efi_loader/Kconfig
@@ -214,6 +214,13 @@
 	  Select this option if you want to enable capsule
 	  authentication
 
+config EFI_CAPSULE_KEY_PATH
+	string "Path to .esl cert for capsule authentication"
+	depends on EFI_CAPSULE_AUTHENTICATE
+	help
+	  Provide the EFI signature list (esl) certificate used for capsule
+	  authentication
+
 config EFI_DEVICE_PATH_TO_TEXT
 	bool "Device path to text protocol"
 	default y