scripts/functions: test for decompressors before use

./configure does check for the presence of gz and bzip2, so we can
safely use them in the build scripts.

On the other hand, more recent formats (eg. XZ) are not yet widely
available, and we do not want, and can't, force the user to install
them as a pre-requisite.

So, build up a list of allowed tarball formats based on the available
decompressors. For no, this is a static list, but the upcoming XZ
support will conditionnaly add to this list.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Dummy config file to not use a C library *at all*

 ## depends on BARE_METAL

 ##

 ## help Do not use a C library.

 ## help

 ## help This is usefull if your project is self-contained, does not rely on

 ## help an external C library, and provides all the necessary bits.

 ## help

 ## help Most probably usefull to bootloaders, as they generally don't depend

 ## help on the C library.

 ## help

 ## help If unsure: do *not* choose that, and use another option in the choice.