Cleanup I2C handling

* Move OS specific code to new files i2c.h and i2c.c
* As side effect a likely bug (linux only code) in the existing driver
  has been fixed.

Compile tested on Debian/i386 and OpenWRT/arm.

Signed-off-by: Harald Geyer <harald@ccbib.org>

server/drivers/i2c.h

@@ -0,0 +1,14 @@
+#ifndef I2C_H
+#define I2C_H
+
+struct i2c_data;
+
+typedef struct i2c_data I2CHandle;
+
+#define I2C_DEFAULT_DEVICE "/dev/i2c-0"
+
+extern I2CHandle *i2c_open(const char *device, unsigned int addr);
+extern void i2c_close(I2CHandle *h);
+extern int i2c_write(I2CHandle *h, void *buf, unsigned int count);
+
+#endif /* I2C_H */