unix/variants: Move setting of MICROPY_PY_USELECT to port config file.

The default is the same as before: MICROPY_PY_USELECT=0 and
MICROPY_PY_USELECT_POSIX=1.  But now this can be easily overridden at the
make command-line using, eg:

    make VARIANT=dev CFLAGS_EXTRA=-DMICROPY_PY_USELECT=1

Signed-off-by: Damien George <damien@micropython.org>

ports/unix/mpconfigport.h

@@ -179,8 +179,11 @@
 #define MICROPY_PY_UHASHLIB_SHA1    (1)
 #define MICROPY_PY_UCRYPTOLIB       (1)
 #endif
+#ifndef MICROPY_PY_USELECT
+#define MICROPY_PY_USELECT          (0)
+#endif
 #ifndef MICROPY_PY_USELECT_POSIX
-#define MICROPY_PY_USELECT_POSIX    (1)
+#define MICROPY_PY_USELECT_POSIX    (!MICROPY_PY_USELECT)
 #endif
 #define MICROPY_PY_UWEBSOCKET       (1)
 #define MICROPY_PY_MACHINE          (1)

ports/unix/variants/coverage/mpconfigvariant.h

@@ -33,7 +33,6 @@
 // Disable some features that come enabled by default with the feature level.
 #define MICROPY_PY_BUILTINS_EXECFILE            (0)
 #define MICROPY_PY_SYS_STDIO_BUFFER             (0)
-#define MICROPY_PY_USELECT                      (0)
 
 // Enable additional features.
 #define MICROPY_DEBUG_PARSE_RULE_NAME  (1)

ports/unix/variants/dev/mpconfigvariant.h

@@ -30,7 +30,6 @@
 // Disable some features that come enabled by default with the feature level.
 #define MICROPY_PY_BUILTINS_EXECFILE            (0)
 #define MICROPY_PY_SYS_STDIO_BUFFER             (0)
-#define MICROPY_PY_USELECT                      (0)
 
 // Enable some additional features.
 #define MICROPY_REPL_EMACS_WORDS_MOVE           (1)

ports/unix/variants/standard/mpconfigvariant.h

@@ -38,7 +38,6 @@
 #define MICROPY_PY_MATH_FACTORIAL               (0)
 #define MICROPY_PY_SYS_PS1_PS2                  (0)
 #define MICROPY_PY_SYS_STDIO_BUFFER             (0)
-#define MICROPY_PY_USELECT                      (0)
 #define MICROPY_PY_UASYNCIO                     (0)
 #define MICROPY_PY_URE_SUB                      (0)
 #define MICROPY_PY_URANDOM_EXTRA_FUNCS          (0)