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>

doc/uImage.FIT/signature.txt

@@ -346,7 +346,9 @@ Simple Verified Boot Test
 
 Please see doc/uImage.FIT/verified-boot.txt for more information
 
+/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
 Build keys
+do sha1 test
 Build FIT with signed images
 Test Verified Boot Run: unsigned signatures:: OK
 Sign images
@@ -355,10 +357,20 @@ Build FIT with signed configuration
 Test Verified Boot Run: unsigned config: OK
 Sign images
 Test Verified Boot Run: signed config: OK
+Test Verified Boot Run: signed config with bad hash: OK
+do sha256 test
+Build FIT with signed images
+Test Verified Boot Run: unsigned signatures:: OK
+Sign images
+Test Verified Boot Run: signed images: OK
+Build FIT with signed configuration
+Test Verified Boot Run: unsigned config: OK
+Sign images
+Test Verified Boot Run: signed config: OK
+Test Verified Boot Run: signed config with bad hash: OK
 
 Test passed
 
-
 Future Work
 -----------
 - Roll-back protection using a TPM is done using the tpm command. This can