[uart] Fix UART driver hangs

The UART driver would constantly hang in unpredictable spots. Turns out
it could get into a situation where it was stuck in a loop unable to
read more from the receive channel, and/or write to the serial port
buffer. Now we use a ring buffer to read as much as possible from the
receive channel, write as much as possible to the serial port buffer,
and move on without looping.

src/libraries/j6/j6.module

@@ -13,6 +13,7 @@ j6 = module("j6",
         "protocol_ids.cpp",
         "protocols/service_locator.cpp",
         "protocols/vfs.cpp",
+        "ring_buffer.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
         "syslog.cpp",
@@ -29,6 +30,7 @@ j6 = module("j6",
         "j6/protocols.h",
         "j6/protocols/service_locator.h",
         "j6/protocols/service_locator.hh",
+        "j6/ring_buffer.hh",
         "j6/syscalls.h.cog",
         "j6/sysconf.h.cog",
         "j6/syslog.hh",