Convert CONFIG_CMD_GSC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_GSC

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/gateworks/gw_ventana/Kconfig b/board/gateworks/gw_ventana/Kconfig
index c577984..5d1bae4 100644
--- a/board/gateworks/gw_ventana/Kconfig
+++ b/board/gateworks/gw_ventana/Kconfig
@@ -14,4 +14,12 @@
 	help
 	  Provides access to EEPROM configuration on Gateworks Ventana
 
+config CMD_GSC
+	bool "Enable the 'gsc' command"
+	help
+	  Provides access to the GSC configuration:
+
+	     gsc sleep - sleeps for a period of seconds
+	     gsc wd - enables / disables the watchdog
+
 endif
diff --git a/configs/gwventana_emmc_defconfig b/configs/gwventana_emmc_defconfig
index 5629dc6..f665cca 100644
--- a/configs/gwventana_emmc_defconfig
+++ b/configs/gwventana_emmc_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_CMD_EECONFIG=y
+CONFIG_CMD_GSC=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
diff --git a/configs/gwventana_gw5904_defconfig b/configs/gwventana_gw5904_defconfig
index 49566ce..c94bedf 100644
--- a/configs/gwventana_gw5904_defconfig
+++ b/configs/gwventana_gw5904_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_CMD_EECONFIG=y
+CONFIG_CMD_GSC=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
diff --git a/configs/gwventana_nand_defconfig b/configs/gwventana_nand_defconfig
index 982a62e..c362be9 100644
--- a/configs/gwventana_nand_defconfig
+++ b/configs/gwventana_nand_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_CMD_EECONFIG=y
+CONFIG_CMD_GSC=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
diff --git a/include/configs/gw_ventana.h b/include/configs/gw_ventana.h
index 0fcbdf9..68e69b8 100644
--- a/include/configs/gw_ventana.h
+++ b/include/configs/gw_ventana.h
@@ -141,7 +141,6 @@
 
 /* Various command support */
 #define CONFIG_CMD_HDMIDETECT    /* detect HDMI output device */
-#define CONFIG_CMD_GSC
 #define CONFIG_CMD_UNZIP         /* gzwrite */
 #define CONFIG_RBTREE
 
diff --git a/include/configs/pcm058.h b/include/configs/pcm058.h
index 041d840..44b4ffd 100644
--- a/include/configs/pcm058.h
+++ b/include/configs/pcm058.h
@@ -87,7 +87,6 @@
 
 /* Various command support */
 #define CONFIG_CMD_HDMIDETECT    /* detect HDMI output device */
-#define CONFIG_CMD_GSC
 #define CONFIG_RBTREE
 
 /* Physical Memory Map */
diff --git a/scripts/config_whitelist.txt b/scripts/config_whitelist.txt
index 129d9a4..c26c6b8 100644
--- a/scripts/config_whitelist.txt
+++ b/scripts/config_whitelist.txt
@@ -383,7 +383,6 @@
 CONFIG_CMDLINE_EDITING
 CONFIG_CMDLINE_PS_SUPPORT
 CONFIG_CMDLINE_TAG
-CONFIG_CMD_GSC
 CONFIG_CMD_HASH
 CONFIG_CMD_HD44760
 CONFIG_CMD_HD44780