build: call ipkg-remove using xargs if #args>=512

build: call ipkg-remove using xargs if #args>=512

The wildcard call to clean up luci package (luci*) can pick up over
2,300 files when the full tree is built. Running make package/luci/clean
or a second run of make package/luci/compile would fail with an
'Argument list too long' error.

To avoid that, a maybe_use_xargs function was created that runs the
command straight as usual if the number of arguments is < 512, or saves
the list in a temporary file and feeds it to xargs otherwise.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>