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/smdk5420_defconfig b/configs/smdk5420_defconfig
index e62f432..a9924a4 100644
--- a/configs/smdk5420_defconfig
+++ b/configs/smdk5420_defconfig
@@ -38,6 +38,7 @@
 CONFIG_ENV_SPI_BUS=1
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_I2C_S3C24X0=y
+# CONFIG_KEYBOARD is not set
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_MMC_DW=y
 CONFIG_MMC_SDHCI=y