Let init create /dev/boringssl/selftest/

This directory is intended to be used by boringssl
(through the bssl_self_test{,64} binaries) to create /
check for the existence of marker files indicating that
the self test has successfully run.

It appears that because this is an .rc script for init
rather than a shell,
  mkdir -p /dev/boringssl/selftest 0755 root root
wouldn't work.

Bug: 139348610
Bug: 136262690
Test: Checked that after booting, /dev/boringssl/selftest
  exists:
  $ su root ls -l /dev/boringssl
  total 0
  drwxr-xr-x 2 root root 40 1972-02-11 03:27 selftest
Test: Checked that if I instead try:
  mkdir -p /dev/boringssl/selftest 0755 root root
  in init.rc then the directory isn't created (there is
  no error message in logcat because logd is only
  started in line 311).

Change-Id: I12fdd08c8ead152ac4e62cbd0a2099a9d6170ddb