flatcar-install uses lbzip2 if present, falls back on bzip2 if not

[aniruddha2000]

Check lbzip2 and set it in global variable

Check if lbzip2 is there or not. If yes then set it to a global variable called BZIP_UTIL or if not then set bzip2 to the global variable and use it for further work.

Testing done

Added lbzip2 to the custom Flatcar image and checked with flatcar-install script flatcar-install -d /dev/vdb (with and without the -f flag).

Writing flatcar_production_qemu_image.img.bz2...
Success! Flatcar Container Linux (from flatcar_production_qemu_image.img.bz2) is installed on /dev/vdb

[pothos]

Looks good, can you squash the commits (and use the message from the second)?

[aniruddha2000]

@pothos yup sure!

[pothos]

Tested with sudo ./flatcar-install -s -o "" -C stable on a QEMU VM and it uses lbzip2 when available and otherwise bzip2, thanks.