upmpdcli: add missing comment dependency propagation

upmpdcli cannot be built with the Blackfin toolchains, but the comment
could be shown with such toolchains, which doesn't make sense. This
commit properly propagates the toolchain exceptions of the package
config symbol to the Config.in comment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>