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>

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"