rpi-config: I2C support

With newer kernels (>=3.18) that supports device-trees I2C
should be enabled with device-trees.

This is now support by adding:
ENABLE_I2C = "1"
in local.conf

This will enable the dtparams:
i2c1
i2c_arm

Signed-off-by: Petter Mabäcker <petter@technux.se>
Signed-off-by: Andrei Gherzan <andrei@gherzan.ro>

README

@@ -27,6 +27,7 @@ Contents:
     3.J. Image with Initramfs
     3.K. Device tree support
     3.L. Enable SPI bus
+    3.M. Enable I2C
 4. Extra apps
     4.A. omxplayer
 5. Board Configuration
@@ -207,6 +208,12 @@ NOTE: KERNEL_DEVICETREE is default enabled for kernel >= 3.18 and always disable
 When using device tree kernels, set this variable to enable the SPI bus
 ENABLE_SPI_BUS = "1"
 
+3.M. Enable I2C
+===============
+When using device tree kernels, set this variable to enable I2C
+ENABLE_I2C = "1"
+
+
 4. Extra apps
 =============