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/docs/getting_started/user-guide.rst b/docs/getting_started/user-guide.rst
index 8915738..1229e66 100644
--- a/docs/getting_started/user-guide.rst
+++ b/docs/getting_started/user-guide.rst
@@ -588,10 +588,8 @@
 
 -  ``KEY_ALG``: This build flag enables the user to select the algorithm to be
    used for generating the PKCS keys and subsequent signing of the certificate.
-   It accepts 3 values: ``rsa``, ``rsa_1_5`` and ``ecdsa``. The option
-   ``rsa_1_5`` is the legacy PKCS#1 RSA 1.5 algorithm which is not TBBR
-   compliant and is retained only for compatibility. The default value of this
-   flag is ``rsa`` which is the TBBR compliant PKCS#1 RSA 2.1 scheme.
+   It accepts 2 values: ``rsa`` and ``ecdsa``. The default value of this flag
+   is ``rsa`` which is the TBBR compliant PKCS#1 RSA 2.1 scheme.
 
 -  ``KEY_SIZE``: This build flag enables the user to select the key size for
    the algorithm specified by ``KEY_ALG``. The valid values for ``KEY_SIZE``