boards/z80/ez80/z20x: Increase RX buffer size to 4Kb, reduce BAUD to 2400 in w25boot configuration
arch/z80/src/ez80/ez80_serial.c: Reduce Rx FIFO trigger level for eZ80F92 to 1 so that will respond more quickly to incoming data.
Squashed commit of the following:
libs/libxx: Fix some confusing in naming. If the directory is called libxx, then the library must be libxx.a (unless perhaps LIBCXX is selected).
libs/: Fix paths in moved library directories.
libs: Brute force move of libc, libnx, and libxx to libs. Cannot yet build it in that configuration.
