nilrt-recovery-media: append ISO IMAGE_FSTYPES
nilrt-recovery-media: append ISO IMAGE_FSTYPES

Any strong assignment to the IMAGE_FSTYPES variable in
nilrt-recovery-media is overwritten at the end of the recipe, when it
includes the `nilrt-core-image.inc` file. As a result, 'wic' is not a
final FSTYPE.

Instead, _append the 'wic' string, so that it is properly added to the
IMAGE_FSTYPES list.

Signed-off-by: Alex Stewart <04cdb2793e4fb0120cc30d1c823099f3862701f4@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt