intel/wifi: Include conditionally in the build

Keep this enabled by default as most x86 platforms could have PCI-e
slots equipped with one of these Intel WiFi adapters.

The Kconfig entries under google boards had no function previously,
the variable was never referenced.

Change-Id: I728ce3fd83d51d4e5e32b848a2079c5fcee29349
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/15931
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/mainboard/google/chell/Kconfig

@@ -28,9 +28,6 @@ config DRIVERS_I2C_GENERIC
 config DRIVERS_I2C_NAU8825
 	def_bool y
 
-config DRIVERS_INTEL_WIFI
-	def_bool y
-
 config IRQ_SLOT_COUNT
 	int
 	default 18

src/mainboard/google/glados/Kconfig

@@ -28,9 +28,6 @@ config DRIVERS_I2C_GENERIC
 config DRIVERS_I2C_NAU8825
 	def_bool y
 
-config DRIVERS_INTEL_WIFI
-	def_bool y
-
 config IRQ_SLOT_COUNT
 	int
 	default 18

src/mainboard/google/lars/Kconfig

@@ -32,9 +32,6 @@ config DRIVERS_I2C_GENERIC
 config DRIVERS_I2C_NAU8825
 	default y
 
-config DRIVERS_INTEL_WIFI
-	default y
-
 config IRQ_SLOT_COUNT
 	int
 	default 18