boot_serial: support fragmentation for outgoing SMP packets

The mcumgr packet sent over serial should be fragmented into frames of
up to 127 bytes, including 2-bytes frame start header and 1-byte for
terminating newline [1], resulting in up to 124 bytes for the base64
encoded payload.

Current implementation doesn't fulfill above requirement and transmits
single frames, without considering their length. This change introduces
support for fragmenting as defined in 'SMP over console' specification.

[1] github.com/apache/mynewt-mcumgr/blob/master/transport/smp-console.md

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>