Add mkfifo2() and pipe2() which are just like mkfifo() and pipe(), but allow control of the size of the underlying, in-memory circular buffer

Adds the simulated QSPI (N25Q) flash to the simulation and modify sim up_spiflash.c to enable it to run with different MTD drivers based on config options (currently m25p, sst26 and w25).

PTY: Fix termios read input processing.. what was I thinking?  Also, if some data was read, pty_read() should not block on the empty FIFO.

This change provides an option to add /dev/urandom to all architectures.  The pseudo-random algorithm I choose strikes an arguably-good balance between being "random" and small/fast enough for 8/16 bit MCUs.  It’s the well-documented xorshift128 algorithm.  It has an internal state of 128 bits that can be [re-]seeded with a write.