Convert CONFIG_KEYBOARD to Kconfig

This converts the following to Kconfig:
   CONFIG_KEYBOARD

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/chromebook_minnie_defconfig b/configs/chromebook_minnie_defconfig
index 41a3fe1..1e87b11 100644
--- a/configs/chromebook_minnie_defconfig
+++ b/configs/chromebook_minnie_defconfig
@@ -63,6 +63,7 @@
 CONFIG_SYS_I2C_ROCKCHIP=y
 CONFIG_I2C_MUX=y
 CONFIG_DM_KEYBOARD=y
+CONFIG_KEYBOARD=y
 CONFIG_CROS_EC_KEYB=y
 CONFIG_CROS_EC=y
 CONFIG_CROS_EC_SPI=y