lib: rsa: decouple rsa from FIT image verification

Introduce new configuration, CONFIG_RSA_VERIFY which will decouple building
RSA functions from FIT verification and allow for adding a RSA-based
signature verification for other file formats, in particular PE file
for UEFI secure boot.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Kconfig b/common/Kconfig
index 46e4193..a2a9b8d 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -1045,3 +1045,10 @@
 endmenu
 
 source "common/spl/Kconfig"
+
+config IMAGE_SIGN_INFO
+	bool
+	select SHA1
+	select SHA256
+	help
+	  Enable image_sign_info helper functions.