make.sh: Fix build with kernels < 6.0.0

make.sh: Fix build with kernels < 6.0.0

unshare(8) seems to fail with kernels older than 6.0.0 when mounting
the /proc filesystem in the inner namespace. This seems to be an bug
where unshare does not even try to mount the /proc filesystem but tries
to make its mount propagation private.

This is now solved in that way that we will use unshare on newer kernels
but will fall back on manually mounting the /proc filesystem once we have
entered the chroot environment.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>