base-files: profile: fix error sh: 1: unknown operand

Handle errors when SHLVL isn't set.

Signed-off-by: Felix Nilsson <felixn@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5df53fcf)
Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>