fs: Convert CONFIG_CMD_CBFS to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_CBFS

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: imply CMD_CBFS on SYS_COREBOOT]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index e0ee8c2..eb70a5f 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -59,6 +59,7 @@
 CONFIG_CMD_REGULATOR=y
 CONFIG_CMD_TPM=y
 CONFIG_CMD_TPM_TEST=y
+CONFIG_CMD_CBFS=y
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_MAC_PARTITION=y
 CONFIG_AMIGA_PARTITION=y