conrtrollers: tcpip + uart: wrappers for read and write

Previously, the actual number of bytes written did not match
what was expected in some cases, in practice. This change
adds a "write all" wrapper around write. Similarly, read did
not always return the full greybus message due to buffering.

In particular, this was the case for devices connected via
6LowPAN which has a very small MTU on IEEE 802.15.4 .

Similarly, a "read one message" wrapper was added
around read that first checks the length of the incoming
greybus message via its header and only reads precisely the
correct number of bytes.

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>