rsa: add sha256-rsa2048 algorithm

based on patch from andreas@oetken.name:

http://patchwork.ozlabs.org/patch/294318/
commit message:
I currently need support for rsa-sha256 signatures in u-boot and found out that
the code for signatures is not very generic. Thus adding of different
hash-algorithms for rsa-signatures is not easy to do without copy-pasting the
rsa-code. I attached a patch for how I think it could be better and included
support for rsa-sha256. This is a fast first shot.

aditionally work:
- removed checkpatch warnings
- removed compiler warnings
- rebased against current head

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: andreas@oetken.name
Cc: Simon Glass <sjg@chromium.org>
diff --git a/test/vboot/sign-images-sha256.its b/test/vboot/sign-images-sha256.its
new file mode 100644
index 0000000..e6aa9fc
--- /dev/null
+++ b/test/vboot/sign-images-sha256.its
@@ -0,0 +1,42 @@
+/dts-v1/;
+
+/ {
+	description = "Chrome OS kernel image with one or more FDT blobs";
+	#address-cells = <1>;
+
+	images {
+		kernel@1 {
+			data = /incbin/("test-kernel.bin");
+			type = "kernel_noload";
+			arch = "sandbox";
+			os = "linux";
+			compression = "none";
+			load = <0x4>;
+			entry = <0x8>;
+			kernel-version = <1>;
+			signature@1 {
+				algo = "sha256,rsa2048";
+				key-name-hint = "dev";
+			};
+		};
+		fdt@1 {
+			description = "snow";
+			data = /incbin/("sandbox-kernel.dtb");
+			type = "flat_dt";
+			arch = "sandbox";
+			compression = "none";
+			fdt-version = <1>;
+			signature@1 {
+				algo = "sha256,rsa2048";
+				key-name-hint = "dev";
+			};
+		};
+	};
+	configurations {
+		default = "conf@1";
+		conf@1 {
+			kernel = "kernel@1";
+			fdt = "fdt@1";
+		};
+	};
+};