Support MODBUS ASCII encoding

[TG9541]

Some application require ASCII encoded MODBUS communication (e.g. MODBUS through telnet).
A MODBUS FC01 "Read Coils" request produced by a jamod test looks like this:

:010100020010EC<CR>

After this request, the receive buffer has the following content (hex):

3A 30 31 30 31 30 30 30 32 30 30 31 30 45 43 0D

Note that the MODBUS over Serial Line specification requires communication events to be terminated by <CR><LF>. The implementation should at least support <CR> but preferably also <CR><LF>.

[TG9541]

The ASC transport, has not been worked on for a long time - and right now I don't see any demand for it. Frankly, why use a transport that transmits 3 times the data and that adds ambiguity about the end of a transmission?

Re-open the issue if there is demand. Until then I will remove UARTASCISR