build-sys: allow per directory build targets

The new --disable-miscutils type of options are meant added for
distribution maintainers who need sometimes only to build util-linux
libraries to bootstrap fresh systems or new architectures.

References: http://www.spinics.net/lists/util-linux-ng/index.html#08695
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
build-sys: add libs-only to config-gen.d

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tools: make config-gen to require build target argument

When argument is not supplied help user by telling what arguments can be
used.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
tests: check commands needed for running a test exist

Signed-off-by: Sami Kerola <kerolasa@iki.fi>