base-files: Avoid problems if DISTRO_VERSION contains undefined variable

base-files: Avoid problems if DISTRO_VERSION contains undefined variable

If DISTRO_VERSION is defined to some undefined variable, e.g.,
DISTRO_VERSION = "${FOO_VERSION}", and /bin/sh is dash, then
do_install() would fail with an error such as:

  run.do_install.2945:193: run.do_install.2945: Syntax error:
  Unterminated quoted string

This was due to unexpanded Python code making it into the shell code,
confusing the shell parser.

Reported-by: srinivasan <srinivasan.rns@gmail.com>
Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>