cert_tool: Support for legacy RSA PKCS#1 v1.5

This patch enables choice of RSA version at run time to be used for
generating signatures by the cert_tool. The RSA PSS as defined in
PKCS#1 v2.1 becomes the default version and this patch enables to specify
the RSA PKCS#1 v1.5 algorithm to `cert_create` through the command line
-a option. Also, the build option `KEY_ALG` can be used to pass this
option from the build system. Please note that RSA PSS is mandated
by Trusted Board Boot requirements (TBBR) and legacy RSA support is
being added for compatibility reasons.

Fixes ARM-Software/tf-issues#499
Change-Id: Ifaa3f2f7c9b43f3d7b3effe2cde76bf6745a5d73
Co-Authored-By: Eleanor Bonnici <Eleanor.bonnici@arm.com>
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
diff --git a/drivers/auth/mbedtls/mbedtls_crypto.mk b/drivers/auth/mbedtls/mbedtls_crypto.mk
index 3819716..21b857b 100644
--- a/drivers/auth/mbedtls/mbedtls_crypto.mk
+++ b/drivers/auth/mbedtls/mbedtls_crypto.mk
@@ -9,7 +9,7 @@
 # The platform may define the variable 'TF_MBEDTLS_KEY_ALG' to select the key
 # algorithm to use. If the variable is not defined, select it based on algorithm
 # used for key generation `KEY_ALG`. If `KEY_ALG` is not defined or is
-# defined to `rsa`, then set the variable to `rsa`.
+# defined to `rsa`/`rsa_1_5`, then set the variable to `rsa`.
 ifeq (${TF_MBEDTLS_KEY_ALG},)
     ifeq (${KEY_ALG}, ecdsa)
         TF_MBEDTLS_KEY_ALG		:=	ecdsa