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

[doak]

Dear Denis,
dear Replicant developers,

I adapted the patches like requested. Additionally I fixed a minor typo in a comment.
Sorry for the long delay.

The original description follows:

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(-)


Best regards,
doak