mii-diag: fix race condition

The mii-diag buildsystem doesn't correctly encode the needed
dependencies, so the install step can race against the mii-diag-udeb
(through all) to either cause mii-diag to be built for the host or
not be available when 'install' is executed.

Work around it by explicitly calling the needed steps and not building
the (unused) udeb version at all.

Fixes http://autobuild.buildroot.net/results/4825d267025c74c522077f84330b47f1110315fa



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>