image: Add RSA support for image signing

RSA provides a public key encryption facility which is ideal for image
signing and verification.

Images are signed using a private key by mkimage. Then at run-time, the
images are verified using a private key.

This implementation uses openssl for the host part (mkimage). To avoid
bringing large libraries into the U-Boot binary, the RSA public key
is encoded using a simple numeric representation in the device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 47ffb44..fed766c 100644
--- a/Makefile
+++ b/Makefile
@@ -247,6 +247,7 @@
 HAVE_VENDOR_COMMON_LIB = $(if $(wildcard board/$(VENDOR)/common/Makefile),y,n)
 
 LIBS-y += lib/libgeneric.o
+LIBS-y += lib/rsa/librsa.o
 LIBS-y += lib/lzma/liblzma.o
 LIBS-y += lib/lzo/liblzo.o
 LIBS-y += lib/zlib/libz.o