sandbox: Enable btrfs support

For better test coverage, enable btrfs.

Signed-off-by: Tom Rini <trini@konsulko.com>

configs/sandbox_defconfig

@@ -60,6 +60,7 @@ CONFIG_CMD_PMIC=y
 CONFIG_CMD_REGULATOR=y
 CONFIG_CMD_TPM=y
 CONFIG_CMD_TPM_TEST=y
+CONFIG_CMD_BTRFS=y
 CONFIG_CMD_CBFS=y
 CONFIG_CMD_CRAMFS=y
 CONFIG_CMD_EXT4_WRITE=y