fix: broken RPCs are left in the receiving end buffer

[eigen-value]

This PR fixes a single issue that randomly happens at restart/reflash if broken RPCs pollute the receiving end.
Printing a short startup message enables the receiving end buffer to flush

[github-actions]

Memory usage change @ 1e5bd1b

Board	flash	%	RAM for global variables	%
arduino:zephyr:unoq	🔺 +52 - +76	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/client flash	%	examples/client RAM for global variables	%	examples/clientSSL flash	%	examples/clientSSL RAM for global variables	%	examples/hci flash	%	examples/hci RAM for global variables	%	examples/monitor flash	%	examples/monitor RAM for global variables	%	examples/server flash	%	examples/server RAM for global variables	%	examples/simple_bridge flash	%	examples/simple_bridge RAM for global variables	%	examples/test flash	%	examples/test RAM for global variables	%	examples/test_rpc_thread flash	%	examples/test_rpc_thread RAM for global variables	%	examples/udp_ntp_client flash	%	examples/udp_ntp_client RAM for global variables	%
arduino:zephyr:unoq	56	0.0	0	0.0	56	0.0	0	0.0	56	0.0	0	0.0	56	0.0	0	0.0	52	0.0	0	0.0	76	0.0	0	0.0	56	0.0	0	0.0	52	0.0	0	0.0	56	0.0	0	0.0

Click for full report CSV

Board,examples/client<br>flash,%,examples/client<br>RAM for global variables,%,examples/clientSSL<br>flash,%,examples/clientSSL<br>RAM for global variables,%,examples/hci<br>flash,%,examples/hci<br>RAM for global variables,%,examples/monitor<br>flash,%,examples/monitor<br>RAM for global variables,%,examples/server<br>flash,%,examples/server<br>RAM for global variables,%,examples/simple_bridge<br>flash,%,examples/simple_bridge<br>RAM for global variables,%,examples/test<br>flash,%,examples/test<br>RAM for global variables,%,examples/test_rpc_thread<br>flash,%,examples/test_rpc_thread<br>RAM for global variables,%,examples/udp_ntp_client<br>flash,%,examples/udp_ntp_client<br>RAM for global variables,%
arduino:zephyr:unoq,56,0.0,0,0.0,56,0.0,0,0.0,56,0.0,0,0.0,56,0.0,0,0.0,52,0.0,0,0.0,76,0.0,0,0.0,56,0.0,0,0.0,52,0.0,0,0.0,56,0.0,0,0.0