tests/pkg_fatfs: Set tarball extension to match content

All existing users treat this as bz2 (tar -j), let's reflect this in the
extension for better usability.

tests/pkg_fatfs/Makefile

@@ -45,7 +45,7 @@ USEMODULE += mtd_sdcard
 endif
 
 image:
-	@tar -xjf riot_fatfs_disk.tar.gz -C ./bin/
+	@tar -xjf riot_fatfs_disk.tar.bz2 -C ./bin/
 
 #this generates a compressed fat image file that can be used by the fat driver on native
 compressed-image:

tests/pkg_fatfs/create_fat_image_file.sh

@@ -13,5 +13,5 @@ sudo mount -o loop,umask=000 riot_fatfs_disk.img /media/riot_fatfs_disk
 touch /media/riot_fatfs_disk/test.txt
 echo "the test file content 123 abc" | tr '\n' '\0' >> /media/riot_fatfs_disk/test.txt
 sudo umount /media/riot_fatfs_disk
-tar -cjf riot_fatfs_disk.tar.gz riot_fatfs_disk.img
+tar -cjf riot_fatfs_disk.tar.bz2 riot_fatfs_disk.img
 rm riot_fatfs_disk.img