Remove RSA PKCS#1 v1.5 support from cert_tool

Support for PKCS#1 v1.5 was deprecated in SHA 1001202d and fully removed
in SHA fe199e3b, 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>