package/pkg-generic.mk: fix show-build-order stdout pollution

The commands like "make show-build-order" or "make
<package>-show-build-order" show the build order and then print
"make[1]: Nothing to be done for 'show-build-order'" to stdout. It
pollutes output. Technically this message is true but it's not true
for user because he gets an information.

The <package>-show-build-order targets use $(info) for package name
printing.  The make utility doesn't consider the internal directive as
a command so it think that it's "Nothing to be done". The patch adds
the empty command to <package>-show-build-order to inform make utility
that taget makes some real actions.

Signed-off-by: Serj Kalichev <serj.kalichev@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: invert $(info) and @:, as suggested by Yann.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>