[Replicant] 'sign-build': Make certficate creation more robust

[doak]

Dear Replicant developers,

in reference to issue [#1870] I made the creation of certificates ('./vendor/replicant/sign-build') more robust.
Please find the patches attached. Let me know if they need some rework to be accepted. Feel free to accept only some of them as well.

    sign-build | 68 ++++++++++++++++++++++++++++++++++++++++++++------------------
    1 file changed, 48 insertions(+), 20 deletions(-)

[#1870] https://redmine.replicant.us/issues/1870

Best regards,
doak