Implement linux-i2c vesion of I2C library for use in Ubuntu platform

Author: @pimvanpelt (mongoose-os-libs/i2c#9)

This adds i2c.dev_no to mos.yml, which instructs the driver to open /dev/i2c-? as backend for I2C.
It then implements a Linux I2C version of the driver, using ioctl() to select the slave, and read/write to write to it.

Notes:

Linux doesn't have a call to free/reset the bus
Linux doesn't expose the underlying SCL/SDA pins, so bitbanging a reset is not possible.
Linux doesn't allow stop bit to be absent (!)
Linux only supports 100KHz frequency.
Basic functionality works -- reading and writing, SMBUS register read/write, and scanning.

CL: ubuntu: Implement linux-i2c vesion of I2C library for use in Ubuntu platform

PUBLISHED_FROM=794b7309c24d382ab9287f09bb9e56b961d6db17

mbedtls/include/mbedtls/version.h

@@ -48,8 +48,8 @@
  *    Major version | Minor version | Patch version
  */
 #define MBEDTLS_VERSION_NUMBER         0x020D0100
-#define MBEDTLS_VERSION_STRING         "2.13.1-cesanta4"
-#define MBEDTLS_VERSION_STRING_FULL    "mbed TLS 2.13.1-cesanta4"
+#define MBEDTLS_VERSION_STRING         "2.13.1-cesanta5"
+#define MBEDTLS_VERSION_STRING_FULL    "mbed TLS 2.13.1-cesanta5"
 
 #if defined(MBEDTLS_VERSION_C)