imx-base.inc: Define a default value for OPTEE_BIN_EXT
Without a default the IMAGE_BOOT_FILES is not parsed properly, leading
to the following error for machines without such a variable definition:
```
| ERROR: _exec_cmd: install -m 0644 -D /z/build-master/imx233-olinuxino-maxi/build/tmp-glibc/deploy/images/imx233-olinuxino-maxi/make_dtb_boot_files /z/build-master/imx233-olinuxino-maxi/build/tmp-glibc/work/imx233_olinuxino_maxi-oe-linux-gnueabi/core-image-base/1.0-r0/tmp-wic/boot.2/make_dtb_boot_files returned '1' instead of 0
| output: install: cannot stat '/z/build-master/imx233-olinuxino-maxi/build/tmp-glibc/deploy/images/imx233-olinuxino-maxi/make_dtb_boot_files': No such file or directory
ERROR: Task (/opt/oe/configs/z/build-master/imx233-olinuxino-maxi/layers/openembedded-core/meta/recipes-core/images/core-image-base.bb:do_image_wic) failed with exit code '1'
```
Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
