utils/check-package: improve shellcheck reproducibility

shellcheck is already in use to check SysV init scripts.
Currently its results can be affected by the existence of a
.shellcheckrc file in any parent directory.

For instance, in this example:
(1) /path/.shellcheckrc
(2) /path/to/.shellcheckrc
(3) /path/to/buildroot
the configs from file (1) are ignored and the configs from file (2)
override the default values from the shellcheck binary.
So the config file affects the check-package result for SysV scripts.

Avoid this reproducibility issue by adding an empty config file to the
buildroot topdir.

It can also eventually contain configs (different from default values
from sheelcheck) that we want as a standard to all shell scripts tested
by check-package.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

DEVELOPERS

@@ -2474,6 +2474,7 @@ F:	package/libhttpparser/
 
 N:	Ricardo Martincoski <ricardo.martincoski@datacom.com.br>
 F:	.flake8
+F:	.shellcheckrc
 F:	package/atop/
 F:	package/thermald/
 F:	support/testing/infra/