base64: Replace the encoder/decoder with low-level functions

base64: Replace the encoder/decoder with low-level functions

The BIO interface is large and very well suited to encode larger
messages. However, we only have very small messages to encode and
therefore will spend a long time on setting up and destroying the BIO
interface all of the time.

These low-level functions are a lot faster and easier to set up.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>