cert_tool: Fix ECDSA certificates create failure

Commit a8eb286a introduced the
following error when creating ECDSA certificates.
    ERROR:   Error creating key 'Trusted World key'
    Makefile:634: recipe for target 'certificates' failed
    make: *** [certificates] Error 1

this patch adds the function to create PKCS#1 v1.5.

Change-Id: Ief96d55969d5e9877aeb528c6bb503b560563537
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>