Convert CONFIG_CMD_HASH to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_HASH

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Rework slightly, enable on some boards again]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/P2020RDB-PC_SDCARD_defconfig b/configs/P2020RDB-PC_SDCARD_defconfig
index 15c9e75..f4cb697 100644
--- a/configs/P2020RDB-PC_SDCARD_defconfig
+++ b/configs/P2020RDB-PC_SDCARD_defconfig
@@ -25,6 +25,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_DATE=y
+# CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
 CONFIG_MTD_NOR_FLASH=y