selinux: enable support for bionic

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

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

Notes:
  matchpathcon selabel_lookup was broken by the incompatible regexec
  which is now in bionic libc (netbsd part), libselinux linkage use
  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)

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

Change-Id: Id82b8a7672a669e3f95360bb70b7b127591f2080