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>

# This file adds functions to build the C library for bare metal (IE. none)

# Copyright 2008 Yann E. MORIN

# Licensed under the GPL v2. See COPYING in the root of this package

do_libc_get() {

    :

}

do_libc_extract() {

    :

}

do_libc_check_config() {

    :

}

do_libc_start_files() {

    :

}

do_libc() {

    :

}

do_libc_finish() {

    :

}