merge_config.sh: ignore unwanted grep errors

The merge_config.sh script verifies that all the config options have
their expected value in the resulting file and prints any issues as
warnings.  These checks aren't intended to be treated as errors given
the current implementation.  However, since "set -e" was added, if the
grep command to look for a config option does not find it the script
will then abort prematurely.

Handle the case where the grep exit status is non-zero by setting
ACTUAL_VAL to an empty string to restore previous functionality.

Fixes: cdfca821 ("merge_config.sh: Check error codes from make")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>