shadow-sysroot: add license information

Recipe references 'login.defs' in LIC_FILES_CHKSUM.  This causes some
problems:

- file does not contain a single word which is related with its license

- changing this file (here: increasing SYS_UID_MIN) invalidates
  LIC_FILES_CHKSUM

Add 'SPDX-License-Identifier' to the file and limit the checksum to
this part.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8c6f2e3f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>