fs: prepare temp directory before running PRE_GEN hooks

fs: prepare temp directory before running PRE_GEN hooks

Some filesystems have PRE_GEN hooks that create a directory structure
under the temporary directory.

For example, iso9660 will create a sub-directory where it stores the
kernel (in case of initramfs or initrd).

So, we must run the PRE_GEN hooks after we cleanup/create the temporary
directory.

Fixes:
    https://gitlab.com/buildroot.org/buildroot/-/jobs/42835965
    https://gitlab.com/buildroot.org/buildroot/-/jobs/42835967

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>