classes/icecc.bbclass: Simplify, improve code creating a tar.bz2

classes/icecc.bbclass: Simplify, improve code creating a tar.bz2
    For icecream we need to package a tar.bz2 containing our
    used toolchain. It must contain a gcc, g++ and cc1 and cc1plus
    and a spec file (if it is not built-in). Optionally it contains
    host libc.so and libfd.so (which we do not package). Simplify the
    code creating the tar.bz2 and make it work with any version of
    gcc/glibc/uclibc (untested).