sanity.bbclass: handle lack of permission to read /proc/sys/vm/mmap_min_addr

sanity.bbclass: handle lack of permission to read /proc/sys/vm/mmap_min_addr

* With 2.6.33-rc2-00252-ge9e5521 on my host I noticed that
  "cat /proc/sys/vm/mmap_min_addr" returns now
  "cat: /proc/sys/vm/mmap_min_addr: Operation not permitted"
  Its probably becuse of
  http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=0e1a6ef2dea88101b056b6d9984f3325c5efced3
  But I'm not sure if checking CAP_SYS_RAWIO even for reading this value
  is intentional or just bug which should be fixed in kernel.

This patch makes bitbake ignore reading error. Possible QEmu failure is not
handled yet.

Acked-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>