tar: add selinux context support on create

No flag is required for the moment, it will add them
to the tar if selinux is enabled on the machine.

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Change-Id: Ic7a39ee03087ed19e814b138ec6d70cdadb605cd
selinux: enable support for bionic, add applets in config

require libsepol package (mm in external/libsepol or add package)

Tested applets:
  getenforce, setenforce, sestatus, selinuxenabled, ls -Z, tar -c,
  getsebool, setsebool,
  runcon, matchpathcon, chcon, restorecon, setfiles

Notes:
  matchpathcon selabel_lookup was broken by the incompatible regexec
  which is now in bionic libc (netbsd part), libselinux linkage used
  busybox one, so a rename of internal regex functions was required

  recovery will not use bb_regex lib, so regex commands could not
  work exactly like xbin or static ones (sed/less/expr/awk/pgrep)
  this is already the case, and can be changed by including libregex
  package in recovery + ifdef in xregex.h

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Change-Id: Id82b8a7672a669e3f95360bb70b7b127591f2080