Remove RSA PKCS#1 v1.5 support from cert_tool

Support for PKCS#1 v1.5 was deprecated in SHA 1001202 and fully removed
in SHA fe199e3, however, cert_tool is still able to generate
certificates in that form. This patch fully removes the ability for
cert_tool to generate these certificates.

Additionally, this patch also fixes a bug where the issuing certificate
was a RSA and the issued certificate was EcDSA. In this case, the issued
certificate would be signed using PKCS#1 v1.5 instead of RSAPSS per
PKCS#1 v2.1, preventing TF-A from verifying the image signatures. Now
that PKCS#1 v1.5 support is removed, all certificates that are signed
with RSA now use the more modern padding scheme.

Change-Id: Id87d7d915be594a1876a73080528d968e65c4e9a
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index f34d3d0..4b83015 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -48,9 +48,9 @@
 					)
 
 # 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`/`rsa_1_5`, then set the variable to `rsa`.
+# 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,
+# then it is set to `rsa`.
 ifeq (${TF_MBEDTLS_KEY_ALG},)
     ifeq (${KEY_ALG}, ecdsa)
         TF_MBEDTLS_KEY_ALG		:=	ecdsa