utils/test-pkg: generate package config if it is not specified

utils/test-pkg: generate package config if it is not specified

It is possible to generate one-line config for the package just by
normalize it to the form:

    BR2_PACKAGE_${pkg_replaced-to_and_uppercase}

it simplifes a bit of testing package where no additional config options
are needed.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>