Disable MMC environment on ASUS Tinkerboard (#3124)

We don't really use the MMC environment, so disable it by default. This
prevents the following warning at startup:
Loading Environment from MMC... *** Warning - bad CRC, using default environment

(cherry picked from commit f263326)

buildroot-external/board/asus/tinker/uboot.config

@@ -1,3 +1,4 @@
 # CONFIG_USB_STORAGE is not set
 # CONFIG_DOS_PARTITION is not set
 CONFIG_OF_LIBFDT_OVERLAY=y
+# CONFIG_ENV_IS_IN_MMC is not set