sstate.bbclass: update the timestamps after install

sstate.bbclass: update the timestamps after install

Update the sstate file's timestamps after it is installed, it will be
very useful for removing the old sstate file, especially, it's not easy
to remove when use the shared SSTATE_DIR, we can easily remove them with
this change, for example:

$ find state-cache -type f -ctime +10 -exec rm -f {} \;

Will remove the sstate file which isn't used by recent 10 days.

We can use the -atime, but it is not always available, for example,
when mounted with "-o noatime".

The touch is a very light weight action, and the
scripts/sstate-cache-management.sh also requires this.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>