Do not write and read rootcerts.crt at the same time

(Possibly) fixes #3470

When updating the main file /files_external/rootcerts.crt we should not
read from /files_external/rootcerts.crt at the same time.

For 2 reasons: writing to a file and reading from it at the same time
can have non deterministic results

And we don't want all the certificates to appear 2 times in there.

This isn't caught by our standard file locking (that does not allow this
actually) because it is in a non locked path....

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>