fs/common: add <fs>_POST_GEN_HOOKS support

The common filesystem infrastructure already supports a
<fs>_PRE_GEN_HOOKS variable, which allows filesystem makefiles to
register some actions to be done before the root filesystem image is
generated.

This commit adds a similiar <fs>_POST_GEN_HOOKS variable, which will
allow filesystem makefiles to do some actions after the filesystem
image has been generated. It will initially be used by the iso9660
filesystem to delete the temporary directory it creates.

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