sstate.bbclass: fix errors about read-only sstate mirrors

sstate.bbclass: fix errors about read-only sstate mirrors

If a read-only sstate mirror is used in conjunction with hash equiv,
then OSError will be raised when an sstate-cache hit is achieved.

This is because sstate_task_postfunc will try to "touch" the symlinks
that point to the read-only sstate mirror when sstate_report_unihash
has changed SSTATE_PKG.

This commit adds an additional exception handler to silently mask read
only rootfs errors thrown during the touch.

The fix is also duplicated to sstate_eventhandler as the code is very
similar but it may not be needed there.

Example of the error:

File: 'exec_python_func() autogenerated', lineno: 2, function: <module>
     0001:
 *** 0002:sstate_task_postfunc(d)
     0003:
File: '/poky/meta/classes/sstate.bbclass', lineno: 774, function: sstate_task_postfunc
     0770:
     0771:    omask = os.umask(0o002)
     0772:    if omask != 0o002:
     0773:       bb.note("Using umask 0o002 (not %0o) for sstate packaging" % omask)
 *** 0774:    sstate_package(shared_state, d)
     0775:    os.umask(omask)
     0776:
     0777:    sstateinst = d.getVar("SSTATE_INSTDIR")
     0778:    d.setVar('SSTATE_FIXMEDIR', shared_state['fixmedir'])
File: '/poky/meta/classes/sstate.bbclass', lineno: 703, function: sstate_package
     0699:    if not os.path.exists(siginfo):
     0700:        bb.siggen.dump_this_task(siginfo, d)
     0701:    else:
     0702:        try:
 *** 0703:            os.utime(siginfo, None)
     0704:        except PermissionError:
     0705:            pass
     0706:
     0707:    return
Exception: OSError: [Errno 30] Read-only file system

Signed-off-by: Michael Ho <Michael.Ho@bmw.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>