Both RXPacket and TXPacket now compute checksums

RXPacket and TXPacket compute checksums before they are converted to buffers and they compare checksums when they are read in from a buffer. If the checksum does not match the transmitted checksum then the packet is reverted to its previous state and read_buffer returns false.

Signed-off-by: Tushar Pankaj <tushar.s.pankaj@gmail.com>
Added flag to check is packet is valid

Signed-off-by: Tushar Pankaj <tushar.s.pankaj@gmail.com>