mk-ca-bundle: detect start of trust section better

Each certificate section of the input certdata.txt file has a trust
section following it with details.

This script failed to detect the start of the trust for at least one
cert[*], which made the script continue pass that section into the next
one where it found an 'untrusted' marker and as a result that certficate
was not included in the output.

[*] = "Hellenic Academic and Research Institutions RootCA 2011"

Bug: http://curl.haxx.se/mail/lib-2012-09/0019.html