zephyr: Switch back to enabling I2C in board-specific configs.

I2C can't be enabled in prj_base.conf because it's a board-specific
feature.  For example, if a board doesn't have I2C but CONFIG_I2C=y then
the build will fail (on Zephyr build system side).  The patch here gets the
qemu_cortex_m3 build working again.

ports/zephyr/prj_base.conf

@@ -14,9 +14,6 @@ CONFIG_NEWLIB_LIBC=y
 CONFIG_FLOAT=y
 CONFIG_MAIN_STACK_SIZE=4736
 
-# Drivers
-CONFIG_I2C=y
-
 # Enable sensor subsystem (doesn't add code if not used).
 # Specific sensors should be enabled per-board.
 CONFIG_SENSOR=y

ports/zephyr/prj_frdm_k64f.conf

 # Networking drivers
 CONFIG_NET_L2_ETHERNET=y
 
+# Hardware features
+CONFIG_I2C=y
+
 # Sensor drivers
 CONFIG_FXOS8700=y
 CONFIG_FXOS8700_MODE_HYBRID=y

ports/zephyr/prj_frdm_kw41z.conf

+# Hardware features
+CONFIG_I2C=y
+
 # Sensor drivers
 CONFIG_FXOS8700=y
 CONFIG_FXOS8700_MODE_HYBRID=y