machine: Add Pin class implementation for Linux.

Tested only for output so far.

machine/machine/__init__.py

@@ -1,4 +1,5 @@
 from .timer import *
+from .pin import *
 
 def unique_id():
     return b"upy-non-unique"

machine/machine/pin.py

@@ -0,0 +1,26 @@
+import umachine
+
+class Pin(umachine.PinBase):
+
+    IN = "in"
+    OUT = "out"
+
+    def __init__(self, no, dir=IN):
+        pref = "/sys/class/gpio/gpio{}/".format(no)
+        dirf = pref + "direction"
+        try:
+            f = open(dirf, "w")
+        except OSError:
+            with open("/sys/class/gpio/export", "w") as f:
+                f.write(str(no))
+            f = open(dirf, "w")
+        f.write(dir)
+        self.f = open(pref + "value", "rw")
+
+    def value(self, v=None):
+        if v is None:
+            return self.f.read(1) == "1"
+        self.f.write(str(v))
+
+    def deinit(self):
+        self.f.close()