ports: Make new ports/ sub-directory and move all ports there.

This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.

ports/pic16bit/unistd.h

@@ -0,0 +1,11 @@
+#ifndef MICROPY_INCLUDED_PIC16BIT_UNISTD_H
+#define MICROPY_INCLUDED_PIC16BIT_UNISTD_H
+
+// XC16 compiler doesn't seem to have unistd.h file
+
+#define SEEK_SET 0
+#define SEEK_CUR 1
+
+typedef int ssize_t;
+
+#endif // MICROPY_INCLUDED_PIC16BIT_UNISTD_H