gen-bios-tar: Update private signing key

Updating from 1024 bits to 4096 bits for a more secure key

Change-Id: Idaa9bcdc04ec4143ff8035c84040646458bf8398
Signed-off-by: Rashmica Gupta <rashmica@linux.ibm.com>
diff --git a/test/utest.cpp b/test/utest.cpp
index 9e55f2a..cdf9b99 100644
--- a/test/utest.cpp
+++ b/test/utest.cpp
@@ -237,7 +237,7 @@
         command("echo \"image-u-boot file \" > " + ubootFile);
 
         std::string pkeyFile = extractPath.string() + "/" + "private.pem";
-        command("openssl genrsa  -out " + pkeyFile + " 2048");
+        command("openssl genrsa  -out " + pkeyFile + " 4096");
 
         std::string pubkeyFile = extractPath.string() + "/" + "publickey";
         command("openssl rsa -in " + pkeyFile + " -outform PEM " +