libtar: store SELinux file context in tar archives

* Using RedHat's "RHT.security.selinux=" tag in extended
  tar header (type 'x')

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>

Change-Id: I102e492e4fa5b8a08fae4d34eb36386862509b4d
Merge "libtar: store SELinux file context in tar archives" into twrp2.7